[Velopack] Create an "Associate Handlers" app

[Geenz]

With velopack and one click install we no longer have a good way to setup the registry. We also lack a good way to uninstall old versions of the viewer on a given channel.

Requirements

• Should check if an old viewer associated with a given channel is installed and trigger its uninstaller
• Must register SLurl handling (if old viewer installed, should uninstall first THEN register)
• Update existing registry entries on Windows
• Should run when the one click viewer runs (just needs to be launched once)

Test Plan

After one click install:

• Check that SLurls work with the one click install (e.g. https://maps.secondlife.com/secondlife/Morris/50/50/39)
• That the old viewer is uninstalled
• That expected registry entries are present

[akleshchev]

Must register SLurl handling

So, for 'native' support it looks like we need to utilize on_after_install() and on_before_uninstall() to set and remove url scheme associations.
Since windows can have only one handler, that means that old one gets unregistered automatically. Need to make sure we are the registered handler before cleanup.
Viewer already processes calls with the url from windows, so that part hopefully works as is.

What nsis does and what needs t be ported:

WriteRegStr HKEY_CLASSES_ROOT "${URLNAME}" "(default)" "URL:Second Life"
WriteRegStr HKEY_CLASSES_ROOT "${URLNAME}" "URL Protocol" ""
WriteRegStr HKEY_CLASSES_ROOT "${URLNAME}\DefaultIcon" "" '"$INSTDIR\$VIEWER_EXE"'

Should check if an old viewer associated with a given channel is installed and trigger its uninstaller

We hold version info per channel in the registry, so it should be possible, something like "Older version x.x.x was detected, do you want to uninstall, you might get a security prompt" as uninstaller will be in a secure location, unlike velopack's install.
Should not be directly relevant to SLurl handling

Update existing registry entries on Windows

Basically the same thing as SLurl handling, just need to look for a channel instead of url sheme keys in the registry.

Should run when the one click viewer runs (just needs to be launched once)

Using velopack's on_after_install() should do that