#50 Allow mouse to be 'locked' within canvas

[William Kendrick]

Provide an option that allows the mouse to be 'locked'
within the canvas (and unlocked later), to help prevent
kids who have difficulty with the mouse from
accidentally clicking tool buttons when they don't want
to. (e.g. accidentally printing while trying to
scribble on the canvas.)

Ref: tuxpaint-users post by Sue Morton, 2005.Jun.03.