GtkTextHandle was neglected by whoever removed the ::draw signal,
leaving it entirely broken. Update to using GtkGizmo so we can
implement snapshot of text handles.

Input has received a revamp too, handling is done through a
GtkGestureDrag and coordinate calculations simplified by storing
the delta to the hotspot on ::begin instead of ::update, as this
value is constant throughout the gesture. Widget state management
on crossing events happens implicitly, so no longer needs to be
done here.

Last but not least, CSS has also been updated so handles are
rendered at the correct size and proportion, and with the padding
that code expects of it.