Button to Toggle Locking Canvas in Place

[thismarty]

Until palm rejection is all osrted out, it sure would be nice to have a button on the toolbar trhat would lock/unlock the canvas position. It's fairly common in my testing to see the canvas shift a little just before I begin a new stroke, triggered by my 'palm' resting on the screen. Thanks!

[Doublonmousse]

That is pretty similar to #1055