Make batch import available through the user interface

[kayaulai]

Right now, batch import is not available through the user interface.

I imagine the way to do it would be: specify a directory with all the import files. Do the import screen for the first file, and assume that all subsequence files will work the same way.

Any errors in the process can be caught instead of crashing Rezonator. If there are failed files, the error message can be displayed in a popup at the end with all the messages for the failed files.

This will probably take a while, but in the long run it means we won't have to ask Terry to do this every time!