FxMixer: Fix channel delete
FxMixer: fix crashes on modifying sends
Added a mutex so that the GUI thread can't change the sends around during the rendering loop.
FxMixer: clearChannel had the same erroneous for loop as deleteChannel
AutomatableModel: prevent linking of model to itself
There's no need to ever link a model to itself, allowing it can cause weird issues and crashes, plus it's easy to do by accident if you start a ctrl-drag and end it while still on the same widget
Merge pull request #846 from diizy/stable-1.1

FxMixer: Fix channel delete and other bugs