provide GUI-settings for luppp.prfs

[georgkrause]

At the moment there are some settings which can only be set by luppp.prfs-file. Would be nice to provide an GUI-Option for these settings.

• saveDirectory
• enablePerTrackOutput
• metronomeActiveByDefault
• metronomeDefaultVolume

[georgkrause]

i forgot:

• enablePerTrackSendReturns