src/pgtkterm.c (x_find_modifier_meanings): Convert virtual super
to non-virtual one.
(pgtk_gtk_to_emacs_modifiers): Use non-virtual one.
(pgtk_emacs_to_gtk_modifiers): Use non-virtual one.
(key_press_event): Don't call pgtk_im_context_filter_keypress while
super is pressed.
src/pgtkterm.h (struct pgtk_display_info): New member.