GtkWindow: Make set_focus work before show

GtkWindow: Make set_focus work before show

So far, gtk_window_set_focus just did not work when called on
a hidden window. Change it to record the desired focus widget
for hidden windows, and apply it when the window gets shown.

This is similar to how we tread other window properties that
can't be set before the window is realized, like maximized
or fullscreen.

This is related to
https://bugzilla.gnome.org/show_bug.cgi?id=734033