This commit is porting GtkPaned to be derived
from GtkWidget instead of GtkContainer, while adding
start-child and end-child properties. The existing
properties are renamed to follow the start/end naming
scheme, and we add proper getters and setters.
Update all users.
See #2719
These complicate a lot of GdkWindow internals to implement features
that not a lot of apps use, and will be better achieved using gsk.
So, we just drop it all.
