Add 2 more variants of seeking shortcuts + hidden config options

Control + L/R - Short seek
L/R - Normal/ Medium seek
Shift + L/R - Long seek

Along this the "Go Up in Media Sources Pane" gets renamed and reassigned
to Backspace.

FEATURES:
* Added keyboard shortcuts for small 2 s seeks (Ctrl + Left/Right) and long 1 minute
seeks (Shift + Left/Right) in addition to standard 10 s seeks. Durations are now
configurable in amarokrc, see Amarok Handbook. Patch by Anmol Ahuja.

@Anmol, you may want to check minor changes I made to the patch.

REVIEW: 108964

FIXED-IN: 2.8
CCMAIL: Anmol Ahuja