add DLS support

[truj]

When #65 is finished, Gervill will be used independently from the Java version. Since Gervill supports DLS (downloadable sounds), it should be easy to implement them in Midica as well.

• [x] rename the soundfont file chooser and enable choosing SF2 and DLS files, distinguishing them by file extension
• [x] support DLS in the soundfont parser
• [x] rename MidicaPL's SOUNDFONT command to SOUNDBANK and support both file types
• [x] make MidicaPL's SOUNDBANK accept URLs as well
• [x] add caching for already downloaded soundbanks
• [x] add a possibility to delete the cache or re-download a cached soundbank

[truj]

Forgot to link this commit: https://github.com/truj/midica/commit/ab5dada9c07b102ee7b5dc42fded693b6ddaea49