[Amarok] rtl awareness, bg gradient, prev/next watermarks +

[Thomas Lübking]

-> discuss it with markey :-P
personally i don't care too much about the gradient at all.

Notes:

- the weird look mostly derives from the round corners of the frames on the 
three lower sections (that oxygen paints on even locked docks)
given there're splitters anyway one could - iff the gradient shall remain - 
one could theoretically force down those frames while the UI is locked.

- please don't revert the entire commit (as it contains various changes and 
introduces vis. strings for next/prev and in case we wanna keep them we're on 
string freeze tomorrow, iirc)

Cheers