WIP: Add extension point for GTK+ modules
GTK+ modules provide extra functionality to GTK+, so this patch adds the extension point to the freedesktop Platform and Sdk runtimes, making it easy for those modules to be used with Flatpak apps.
Extensions that have GTK+ modules should have an org.gtk.Gtk3module as a prefix in their name. Note that in order for the actual modules to be loaded, they have to be added to the GTK3_MODULES environment variable (passed Flatpak when running an app).
See #366 (closed) for motivation and discussion.
Edited by Joaquim Rocha