Minor whitespace consistency changes
Menus: Use the win. group/prefix for Cut/Copy/Paste as Print menu items.

Because Why Not. It is what the GTK+ test/example code does, and might
have some (undocumented, unexplained) advantage.

* glom/modedesign/print_layotus/window_print_layout_edit.[h|cc]:
  Change this to a Gtk::ApplicationWindow so we can use its
  regular add_action() method.
* glom/appwindow.cc: For the Edit menu's Cut, Copy, Paste, Delete,
  Select and Select All items, and for the File menu's Print Layout
  and Print Preview, just use the regular "win" action group.
* ui/developer/window_print_layout_edit.glade: Change this to
  a GtkApplicationWindow.
* ui/operator/window_main.glade: Use the win. prefix for some
  actions.
Print Layouts: Avoid crash when opening.

* glom/frame_glom.cc: This was another ! error from the commits to
  avoid clang warnings.