Add standard cmd+scroll / ctrl+scroll to zoom

[sanj0]

As is standard many tools (gimp, some text editors, web browsers, etc.), cmd+scroll or ctrl+zoom (in windows / linux) is a nice way to zoom in and out.

Holding down cmd (ord ctrl) and then scrolling should zoom in and out. It's faster in two ways:

1. When drawing / editing pixels, you use your mouse the whole time, so it's faster and more convenient to not have switch the right hand from mouse to plus/minus keys (which are on the right side of the keyboard)
2. Scrolling itself is faster and more precise

"Scroll Zoom" is a non-optimal solution for mouse users, as for one it's non-standard (as far as I'm concerned) and secondly because it has the potential to not work (as it does for me, reported in #176).

Possible dublicate of #93