BSlider: support mouse wheel and snap to hash marks with SHIFT

[humdinger]

Hard working dev dsizzle is improving ArtPaint while I try out his changes and give feedback. We found a few things that could be improved for BSliders. I'll leave more technical features for him to report, but I'd like to mention two issues from a user perspective:

1. It'd be nice to control a BSlider with the mouse wheel when you hover over it. It gives more fine-grained control that moving the thumb with the the mouse.

2. Hash marks are nice to indicate 'special' values, like angles of rotation (90 °,45°, 0°). However, it's not easy to hit those exact values by moving the thumb (the mouse wheel would help to fine-adjust here, too). How about snapping the thumb to the hash marks when holding SHIFT?

Can that be built-in to BSlider or would it have to be implemented in every app?

[waddlesplash]

1 can be done, for sure. I think 2 as well, indeed.