Don't hook on the widget style context and set up instead
a widget path for itself. Also use a common style class
for both handles, with an extra top/bottom class for each
handle.

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.