this cleans up the slider a bt and makes it rtl capable

done:
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- right-to-left support for the custom slider
- !!! VISUAL change: don't draw a custom knob if disabled (it's currently only used and relevant for the timeslider. this stresses "you cannot drag" while playing streams)
- moodbar swapping
- some code cleanup, notably
* the weird "adjustedValue()" thing is gone. the inversion is handled by a flag and not by swapping around values ;-)
* use QStyleOptionSlider for painting. It keeps req. state flags.

"to do":
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- svg sking completion:
the RTL slider needs an additional svg element (counterpart of the sliderstart)
as merging the svg is pretty nasty and i forsee a change on the skip icons, i'll preferably do that from master as soon as and in case this is merged ;-)