converted all foreign controls to ControObjects Slave in BpmControl

This ensures that no update missed from other engine controls in case of later changes.
Merge remote-tracking branch 'mixxx/master' into master_sync

Conflicts:
	src/engine/bpmcontrol.cpp
	src/engine/bpmcontrol.h
Currently not-working attempt to mock a playing deck.  Problem:
The enginebuffer creates a bunch of objects that I can't avoid.
Refactor mocked backend testing stuff
Playing test works!
remove tabs