Depending on the OS, the display name may contain characters which are not valid
for action names. In particular, on X11, the display name could be ":0.0" and
therefore we get actions named "app.dock-move-to-screen-:0.0" or
"app.view-move-to-screen-:0.0". The ':' is invalid, which will make calling
gtk_application_get_accels_for_action() (or more simply our new function
gimp_action_get_accels()) fail.
See docs of g_action_parse_detailed_name().
The new utils function gimp_make_valid_action_name() will help us get rid of
invalid characters for actions whose name was generated from strings we don't
totally control.
Furthermore gdk_screen_make_display_name() is now deprecated API, and more
generally gdk_display_get_n_screens() is deprecated too and now always returns 1
since GTK+ 3.10 (i.e. now each display only contains a single screen). Instead
we just use gdk_display_get_default_screen() for every GdkDisplay. So this
commit doubles as GTK/GDK deprecation cleanup.
fd7ec4df6a