AppDisplay: clean up handling of right click popup menu

Instead of connecting a global activate handler on the menu,
have one on each menu item, individually doing the right thing.

https://bugzilla.gnome.org/show_bug.cgi?id=669603
Introduce support for desktop actions in the dash

Using the new list_actions() API in Gio, add entries for static
actions specified in .desktop files in the right-click app menus,
in the dash, app well and search.

https://bugzilla.gnome.org/show_bug.cgi?id=669603
ShellApp+ShellGlobal: unify app launch context code

Extend shell_global_create_app_launch_context() with the required
parameters and use that for shell_app_launch() too.

https://bugzilla.gnome.org/show_bug.cgi?id=669603