Added Vertical Slider styles to Slider widget

Added WIDGETSTYLE2 (like 0, but vertical) and WIDGETSTYLE3 (like 1, but vertical). NOTE: I don't have a build environment, so I can't even promise this will compile! So if it's crap, just treat it as a concept for an enhancement request.

Also, please be gentle -- this is my virgin attempt to submit code to somebody else's Git repository, so if I'm doing the whole branch/fork/pull thing incorrectly, please let me know so I will know better next time.