Support multiple simultaneous decompilations

[PeterMatula]

Right now, only one decompilation can run at a time. When second decompilation is triggered before the first one is finished, the first one gets killed and the second one takes its place.

This could be made better by supporting multiple decompilations at one time. There is no principal problem with this. It is possible to run the decompilation script multiple times at one moment. However, the whole thing is not trivial. We would have to make sure these decompilations are not using the same resources - both files on the disc, and internal structures in RetDec plugin. All related structures and actions (e.g. navigation) would have to be duplicated and associated with a single decompilation.