Phantom column in library table ---(Bug #1258776) fixed

KEY_ID is only needed to cache the global key and like all internal
housekeeping variables in the database it is internal.
Fixed compiling VsyncThread with Qt5
Fix compiling MixxxApplication wit Qt5
Don't hard-code LoadSelectionToGroup.  We already have a CO, [ChannelX],LoadSelectedTrack.
Remove unnecessary code which breaks compile on other than linux.

The thread cpu timer is adopted from performance timer code for
linux. There is no implementation for other platforms, but the old
functions were left around leading to duplicate symbols when linking.

Removed these and renamed 'default' dummy functions.
Removed unused function
Added Khyrul Bashar to contributor list. Thank you!