support importing mixed directories of roms

[anderbubble]

Importing a large rom collection currently requires a bit of manual intervention, where each platform must be imported individually. A single directory hierarchy with multiple platforms should be importable as a single operation, with platform auto-detected at import time.

[anderbubble]

What's more, you can't queue up multiple import operations; so you have to wait for each import to complete before you can start the next. At the very least it would be nice to be able to queue up multiple import jobs so I could set it and just let it run.