Implement power management: suspend inhibition and pause on resume

FEATURES:
  * Added options to pause playback on system suspend and to inhibit automatic suspend
    if playing a track (enabled by default); patch by Anmol Ahuja. (BR 259862, 222571)

FEATURE: 259862
FEATURE: 222571
FIXED-IN: 2.8
REVIEW: 109846
DIGEST: Amarok is now power-management aware: it can inhibit automatic system suspend
and pause on resume (configurable)