enable import/export from/to foreign formats using foreign tools

[truj]

Import: Call foreign command line tools to import from their file using a temporary MIDI file. Then import this file.

Export: Export to a temporary MIDI file. Then call the foreign tool to convert to their format.

Use cases:

• Import ALDA or Musescore files directly.
• Use Midica as a GUI tool for ALDA development (like it's done with MidicaPL files)
• Use Musescore to export to one of their supported export format (Musescore, SVG, PDF etc.)

TODOs:

• Reorganize the import/export area:
• One button for all imports, one button for the soundfont, one button for all exports.
• Use tabs in the import file chooser like it's already planned for exports.
• Create a class for foreign imports/exports.
• Handle exceptions like program "not installed", "not found", etc.
• Enable to choose the executable path, if it's not in the path variable