Add a new StPasswordEntry widget

We are moving to new dialogs in Cinnamon and several contain password
entries. This gives a new entry to use for those dialogs that features a
a show/hide password feature

Author: JosephMcc

data/icons/hicolor_actions_scalable_view-conceal-symbolic.svg

@@ -56,6 +56,11 @@ stage {
   StLabel.hint-text { color: transparentize($fg_color, 0.7); }
 
   .capslock-warning { icon-size: $scalable_icon_size; }
+
+  StIcon.peek-password {
+    icon-size: 16px;
+    color: $fg_color;
+  }
 }
 
 // buttons in dialogs

data/icons/hicolor_actions_scalable_view-reveal-symbolic.svg

@@ -15,6 +15,7 @@ st_headers = [
     'st-icon-colors.h',
     'st-label.h',
     'st-image-content.h',
+    'st-password-entry.h',
     'st-polygon.h',
     'st-private.h',
     'st-scrollable.h',
@@ -114,6 +115,7 @@ st_gir_sources = [
     'st-icon-colors.c',
     'st-image-content.c',
     'st-label.c',
+    'st-password-entry.c',
     'st-polygon.c',
     'st-private.c',
     'st-scrollable.c',