libs: vaapiwindow: Refactor into GstObject

This of course also implies refactoring its subclasses.

Note that this gives us several important features:

• We can send signals from a window (such as a "size-changed" signal) to another object. This allows for example the vaapisink to listen to configure signals in wayland.
• By defining GstVaapiWindow as an abstract type, we can prevent someone from instantiating it (and it is clear to contributors as such).
• Rather than defining "helper macros" that blindly cast their arguments, we can rely on functions such as g_object_get () to fetch properties.

This is a follow-up for #96 (closed)