Make it possible to enqueue conversions

When converting a file via "Open Multiple Files.." it will add the conversion to the playlist, but will start it immediately ending any running conversion prematurely. It would be great, if the conversion could have something like an enqueue flag, so you could setup multiple different conversions while not waiting for the current one to finish.

Right now it can be used for one-off conversions, but not really for batch processing.

And using the command-line strangely requires information about the input file, which the UI doesn't, so scripting is not really feasible for batch conversions either.