#788 Button bindings for "increase rotation by 90 degrees" and "decrease rotation by 90 degrees"

[cheater]

I could really, really use that.

Please beari n mind that there is this related bug in existing rotation logic:
https://sourceforge.net/p/smplayer/bugs/1007/

The new function should respect currnet zoom, pan, mirroring, and aspect ratio settings

Thank you!