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.
This is a helper object to allow text widgets to implement
text selection on touch devices. It allows for both cursor
placement and text selection, displaying draggable handles
on/around the cursor and selection bound positions.
Currently, this is private to GTK+, and only available to
GtkEntry and GtkTextView.
