Add "What's New" page to show release notes after update to new version

Currently when a user uses Sparkle updater to update, they will see the
release notes beforehand. However, if they are updating across multiple
versions, they only see the latest one. Also, if they are using
automatic update, they will not see the release notes page. Users who
get MacVim from Homebrew or manually building from source also do not
see the release notes as those mechanisms are driven from command line.
This makes it harder to communicate new features and announcements to
these users.

Add a new "What's New" page that will be automatically shown whenever
the user has updated to a new version of MacVim. The last version of
MacVIm is tracked by the MMLastUsedBundleVersion value previously added
in #1357, which allows us to detect such update no matter what
installation method was used and display a dialog box. Other than
opening at launch, the user can also open it in the Help menu, and
there's an option to make it not open at launch if it's annoying to
the user.

The release notes is served by http://macvim.org/ (done in
macvim-dev/macvim-dev.github.io#1) so we don't
have to bundle it locally and it makes it easier to update them. MacVim
will know the request a range of release notes (if updating across
multiple versions at once) so that all new versions will be visible.

Also, fix it so that both Sparkle updater and the new What's New page
will properly be shown on top of the new MacVim window that gets opened
when MacVim is launched. Previously when we get a new update, Sparkle
frequently gets hid behind the editor window.

Author: ychin

runtime/doc/gui_mac.txt

@@ -308,6 +308,7 @@ KEY				VALUE ~
 *MMVerticalSplit*		files open in vertical splits [bool]
 *MMZoomBoth*			zoom button maximizes both directions [bool]
 *MMUpdaterPrereleaseChannel*	opt-in to pre-release software update [bool]
+*MMShowWhatsNewOnStartup*	show "What's New" after updating to new version [bool]
 
 As an example, if you have more than one mouse button and would wish to free
 up Ctrl-click so you can bind it to something else, then the appropriate

runtime/doc/tags

@@ -5520,6 +5520,7 @@ MMNonNativeFullScreenShowMenu	gui_mac.txt	/*MMNonNativeFullScreenShowMenu*
 MMRendererClipToRow	gui_mac.txt	/*MMRendererClipToRow*
 MMShareFindPboard	gui_mac.txt	/*MMShareFindPboard*
 MMShowAddTabButton	gui_mac.txt	/*MMShowAddTabButton*
+MMShowWhatsNewOnStartup	gui_mac.txt	/*MMShowWhatsNewOnStartup*
 MMSmoothResize	gui_mac.txt	/*MMSmoothResize*
 MMTabMaxWidth	gui_mac.txt	/*MMTabMaxWidth*
 MMTabMinWidth	gui_mac.txt	/*MMTabMinWidth*

runtime/lang/macvim_menu/menu_ca_es.latin1.apple.vim

@@ -25,5 +25,6 @@ menutrans Zoom\ All Fer\ zoom\ a\ tot
 menutrans Show\ Next\ Tab Mostrar\ la\ pestanya\ següent
 menutrans Show\ Previous\ Tab Mostrar\ la\ pestanya\ anterior
 menutrans Bring\ All\ to\ Front Portar-ho\ tot\ a\ primer\ pla
+menutrans What's\ New Novetats
 menutrans Release\ Notes Notes\ de\ la\ versió
 menutrans Look\ Up Consultar

runtime/lang/macvim_menu/menu_cs_cz.utf-8.apple.vim

@@ -25,5 +25,6 @@ menutrans Zoom\ All Přepnout\ velikost\ všech
 menutrans Show\ Next\ Tab Zobrazit\ další\ panel
 menutrans Show\ Previous\ Tab Zobrazit\ předchozí\ panel
 menutrans Bring\ All\ to\ Front Převést\ vše\ do\ popředí
+menutrans What's\ New Novinky
 menutrans Release\ Notes Poznámky\ k\ vydání
 menutrans Look\ Up Vyhledat

runtime/lang/macvim_menu/menu_da.utf-8.apple.vim

@@ -25,5 +25,6 @@ menutrans Zoom\ All Zoom\ alle
 menutrans Show\ Next\ Tab Vis\ næste\ fane
 menutrans Show\ Previous\ Tab Vis\ forrige\ fane
 menutrans Bring\ All\ to\ Front Anbring\ alle\ forrest
+menutrans What's\ New Nyheder
 menutrans Release\ Notes Frigivelsesnoter
 menutrans Look\ Up Slå\ op

runtime/lang/macvim_menu/menu_de_de.latin1.apple.vim

@@ -25,5 +25,6 @@ menutrans Zoom\ All Alle\ zoomen
 menutrans Show\ Next\ Tab Nächsten\ Tab\ anzeigen
 menutrans Show\ Previous\ Tab Vorherigen\ Tab\ anzeigen
 menutrans Bring\ All\ to\ Front Alle\ nach\ vorne\ bringen
+menutrans What's\ New Neue\ Funktionen
 menutrans Release\ Notes Aktuelle\ Informationen
 menutrans Look\ Up Nachschlagen

runtime/lang/macvim_menu/menu_es_es.latin1.apple.vim

@@ -25,5 +25,6 @@ menutrans Zoom\ All Aplicar\ zoom\ a\ todas\ las\ ventanas
 menutrans Show\ Next\ Tab Mostrar\ pestaña\ siguiente
 menutrans Show\ Previous\ Tab Mostrar\ pestaña\ anterior
 menutrans Bring\ All\ to\ Front Traer\ todo\ al\ frente
+menutrans What's\ New Novedades
 menutrans Release\ Notes Notas\ de\ la\ versión
 menutrans Look\ Up Consultar

runtime/lang/macvim_menu/menu_fi_fi.latin1.apple.vim

@@ -25,5 +25,6 @@ menutrans Zoom\ All Zoomaa\ kaikki
 menutrans Show\ Next\ Tab Näytä\ seuraava\ välilehti
 menutrans Show\ Previous\ Tab Näytä\ edellinen\ välilehti
 menutrans Bring\ All\ to\ Front Tuo\ kaikki\ eteen
+menutrans What's\ New Uutta
 menutrans Release\ Notes Julkaisutiedot
 menutrans Look\ Up Katso\ lisää

runtime/lang/macvim_menu/menu_fr_fr.latin1.apple.vim

@@ -25,5 +25,6 @@ menutrans Zoom\ All Réduire/agrandir\ toutes\ les\ fenêtres
 menutrans Show\ Next\ Tab Afficher\ l’onglet\ suivant
 menutrans Show\ Previous\ Tab Afficher\ l’onglet\ précédent
 menutrans Bring\ All\ to\ Front Tout\ ramener\ au\ premier\ plan
+menutrans What's\ New Nouveautés
 menutrans Release\ Notes Notes\ de\ mise\ à\ jour
 menutrans Look\ Up Définition

runtime/lang/macvim_menu/menu_hu_hu.utf-8.apple.vim

@@ -25,5 +25,6 @@ menutrans Zoom\ All Összes\ nagyítása
 menutrans Show\ Next\ Tab Következő\ lap\ megjelenítése
 menutrans Show\ Previous\ Tab Előző\ lap\ megjelenítése
 menutrans Bring\ All\ to\ Front Összes\ előtérbe\ hozása
+menutrans What's\ New Újdonságok
 menutrans Release\ Notes Kibocsátási\ megjegyzések
 menutrans Look\ Up Definiálás