(cherry-picked to fixAppleCommander/AppleCommander#13 since I've
discovered that I was wrong and it does fix the problem after all!)
cwa.storage.Disk.FileFilter was written for SWT, returning multiple
extensions as a single string with ; separators. For JavaFX, we
actually want individual strings separately or in a list.
The only place these are initialized in cwa.storage.Disk anyway, so what
I've done is change the class to instatiate with separate strings and
join them on demand for SWT. A new getExtensionList() method will
return them as-is for JavaFX.