gdkwindow: Handle references in "update_windows" list correctly

gdkwindow: Handle references in "update_windows" list correctly

Since update_windows list is a static variable in GdkWindow.c which
contains pointers to windows which needs to be updated, it can happen
that it contains a pointer to a window even after quit from a gtk_main().
If another gtk_main() is called in the same process it tries to process
windows in the list which leads to a crash.
Correct reference count handling of added windows prevents such applications
from crash.

https://bugzilla.gnome.org/show_bug.cgi?id=711552