gtk4: Fix segfault running `gst-inspect -a` when gtk4 and gtk3 is installed
Segmentation fault when getting default value of paintable property from gtk4paintablesink element when libgtk-4.so.1 from libgstgtk4.so and libgtk-3.so.0 from libgstgtk.so are installed:
cannot register existing type 'GdkDisplayManager'
Fixes #490 (closed)