sessionMode: Add allowKeybindingsWhenModal property

Add a sessionMode.allowKeybindingsWhenModal property, which determines
whether keybindings should still be handled while a modal dialog is
up or not.

https://bugzilla.gnome.org/show_bug.cgi?id=676156

js/ui/sessionMode.js

@@ -13,6 +13,7 @@ const _modes = {
              showCalendarEvents: false,
              allowSettings: false,
              allowExtensions: false,
+             allowKeybindingsWhenModal: true,
              sessionType: Shell.SessionType.GDM },
 
     'user': { hasOverview: true,
@@ -20,6 +21,7 @@ const _modes = {
               showCalendarEvents: true,
               allowSettings: true,
               allowExtensions: true,
+              allowKeybindingsWhenModal: false,
               sessionType: Shell.SessionType.USER }
 };