GtkColorEditor: Keep ref on pre-popup focus widget

GtkColorEditor: Keep ref on pre-popup focus widget

And ensure it's still visible before returning the keyboard focus to it.
Because of the extra ref, add a dispose handler that will ensure the
ref is lost (by popping down), although this should be already ensured
through other paths (eg. when the popup widget loses visibility).

This fixes a possible crash in dispose paths, where we might be restoring
focus on an already destroyed widget, and at a time where, if the toplevel
is being itself disposed, no new focus should be set.

https://bugzilla.gnome.org/show_bug.cgi?id=767849