Support scroll events in Mac OS

[ingo-dsp]

I noticed that scroll events seem to be ignored when using baseview on Mac OS. I can reproduce this problem with my VST plugin in Cubase on Mac OS.

Therefore I had a look at the code and it seems to me that scroll events are not handled at all in src/macos/view.rs.

I created the pull request #110 that adds this functionality there. If you agree that this functionality was missing before, feel free to use my code, or use it as a starting point if you like.