Improve mouse selection peformance by throttling shadow cursor redrawing

Drawing the cursor on every mouse move event badly impacts the responsiveness
of the application, because the move events buffer in the browser until
every single event is processed. By setting these on a background timer,
the cursor is updated asap, but is able to aggregate multiple move events
received in this time period.
Merge pull request #167 from peitschie/delay-cursor-draw

Improve mouse selection peformance by throttling shadow cursor redrawing