Set gtk-modules to auto-load appmenu-gtk-module

Summary:
Makes it work out of the box for both GTK 2 and GTK 3 applications.
Since both krdb and kde-gt-config overwrite their respective gtkrc-2.0 we use a watcher to re-add our config key afterwards.

Test Plan:
Tested Pluma (Gtk3), now has global menu
Tested Gimp and Inkscape, now have global menu
When module isn't installed, prints a warning on console, shouldn't be too bad

Reviewers: Plasma, davidedmundson, fvogt, hein

Reviewed By: Plasma, hein

Subscribers: cgiboudeaux, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15315