Proposed fix for 1352  Ifreeze when adding instruments or moving mixer tracks

Proposed fix for #1352  Ifreeze when adding instruments or moving mixer tracks

The Instruments we being loaded on the main thread, and causing deadlock with MixerWorkerThread.
I have moved the loading of instruments from the gui into a separate thread, thus avoiding the deadlock.

You may want to check i have done this right, as I am out of my comfort zone.