Call AudioOutput::onOutputConnected and AudioOutput::onOutputDisconnected on
device open and close.

We do this for AudioInput::onInputConnected/AudioInput::onInputDisconnected but
apparently never finished implementing the other half :).
Add simple tracking of connected AudioOutputs in EngineMaster when outputs are
connected and disconnected.
Fix race condition in clearing leaked controls.
Fix some uninitialized variables discovered using valgrind.
Don't change the scaler if it is over-ridden for testing.
Only force vinyl mode to absolute on seek if the deck is playing.
Merge branch 'master' into cdj

Conflicts:
	src/mixxx.cpp
Fix segfault at Mixxx shutdown