application: Create the main window after the gear-menu action

PhotosMainWindow creates PhotosMainToolbar which uses the gear-menu
action.

Fallout from c44d891f37488f1909233c032b657c2754b4e2ab

Fixes: https://bugzilla.gnome.org/708018
Utilize intltool 0.50 gsettings support

This lets us replace @INTLTOOL_XML_NOMERGE_RULE@ with
@INTLTOOL_XML_RULE@ so that we can merge the appdata translations into
the installed file

https://bugzilla.gnome.org/show_bug.cgi?id=708054