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
The position child property is problematic, since it
requires us to emit notification for all children when
inserting a child early in the list of children.
Remove the property from all ui files.
