Detect wheel event on canvas and post data to VM
Detect wheel event on canvas and post data to VM

This, along with the related VM pull request, fixes LLK/scratch-vm#865.
This adds the GUI code which posts wheel scroll data to the VM's
mouseWheel IO (which, in turn, activates relevant "when key pressed"
blocks).

cwillisf/scratch-gui,cwillisf/scratch-gui,cwillisf/scratch-gui,LLK/scratch-gui,LLK/scratch-gui