[screenlocker] Support global shortcuts in the lock screen

This change implements support for white listed global shortcuts in
the lock screen. It interacts with KGlobalAccel to fetch shortcuts
and checks them when a key is pressed. For more detailed information
on how this functions, please see the documentation added to the new
file globalacel.h.

So far only shortcuts from kmix are white listed. This allows to
mute and change volume while the screen is locked.

CCBUG: 148228
CCBUG: 104353
FEATURE: 198097
FIXED-IN: 5.3.0
REVIEW: 122419