Generalize IndexCreatorDialog to work with other file types

So far have only implemented VCF, and haven't put that into the UI yet (should in theory work but doesn't get called).

IGV-1969
Add additional tests for CommandExecutor/Listener

Add tests of url-encoding/decoding. Add tests for file URLs
Tweak CommandExecutor/CommandListener

File parameter should be URL decoded at most once now, and then only if a URL or if passed through CommandListener (via http link, MUST be url-encoded in that case)
Add basic test