Start stability tests:
 - randomly add / remove widgets and graphicsitems to provoke a crash
 - create and delete various objects and ensure that nothing is left in memory
stability test is successfully crashing, ref_cycle test exposes cycles!
Fixed ref cycle in SignalProxy
update stability test
Expanded ref checks
Fixed ref cycle in ImageItem -> HistogramLutItem
Expand ref cycle check to include all child QObjects
Add GraphicsWindow ref check
disabled stability test (because it is expected to crash)
Merge branch 'stability_tests' into develop