gtktexthandle: Refactor and use native surfaces

gtktexthandle: Refactor and use native surfaces

Instead of being a GObject managing two GtkWidgets, make GtkTextHandle
a GtkWidget subclass, representing a single handle.

From the perspective of users (GtkText and GtkTextView), this is not a
big leap since they have to be aware of a great deal of text handles'
state. It actually makes things more direct and simple.

With text handles being widgets, those can be actual children of the
widget, and may have their own GdkSurface that we move around at will.
This is the second major aspect of this refactor.