Issue #209: Configurable Behavior of FF, REW, NEXT & PREV

Add settings to control how these buttons work.
FF & REW can be configured to skip 10 sec, 30 sec, 1 min or 3 min.
NEXT & PREV can be configured to either skip 5%, 10%, 5 min and 10 min or retain their old behavior of NEXT skipping to next item on queue and PREV doing the same as REW
Merge pull request #216 from alonalbert/NextPrev

Issue #209: Configurable Behavior of FF, REW, NEXT & PREV