GtkMenuTracker: set action namespace correctly

gtk_menu_tracker_add_items() fetched the action-namespace from the menu
item, but didn't pass it into gtk_menu_tracker_section_new() when its
internal namespace was still NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=712164
GtkApplicationWindow: avoid mis-allocation

In the fallback case, we were adding the border width of the
window twice, causing the content to be mis-positioned and
cut off in some cases.

https://bugzilla.gnome.org/show_bug.cgi?id=710909