Improved mouse support

Added more mouse support functionality, including the ability to discriminate
which mouse button was pressed as well as reporting click-drags, and mouse
movements when no mouse button is pressed. Also improved mouse support while
text editing: when you click and drag, your selection will be shown in realtime
(as opposed to having to release the mouse press to see the resulting
selection).

To see this functionality in action, try running
examples/full-screen/text-editor.py, write some text, then click and drag over
that text to select it.

Commit co-authored by: Jonathan Slenders