Allow to change the brush-size with the mouse-wheel

[simonbethke]

I implemented a hidden but useful feature that allows to adjust the brush-size with the mouse-wheel by holding down the shift-key.

[simonbethke]

Actually, this even fixes a bug that is still existent for other selection-modes:

While selecting, you can zoom in and out with the mouse-wheel. I don't think this is how it is supposed to work. Needs to be fixed for rectangle and poly (and lasso) selection too.

And this is the part where I base-class would become handy. I have the impression, that most of my last additions could benefit from the suggested refactoring.