Disable draw button while panning (or zooming)

[Zacrain]

Feature/enhancement description:
I'm using a graphic tablet and have set panning on one of the tablet buttons (which basically maps to middle mouse button). Since it may unintentionally happen that the pen's tip comes in contact with the tablet's surface when panning, it will simultaneously draw, as if holding middle mouse button and left mouse button at the same time.
In consequence I have to hit undo often or use the eraser more times than I would like to.

Therefore it would be great to disable being able to draw while panning or zooming. Adding that as an option for users who would like to retain the old behaviour for some reason would suit both user groups.

[Zacrain]

I took the liberty to solve this problem myself. 😁 See PR #302 .