Add application preferences/settings window

[will2dye4]

The app should have configurable preferences for things like:

• # of recently opened tracks to display in the File menu (+ option to clear recently opened tracks?)
• default output directory for isolated tracks
• default audio format for isolated tracks (MP3 or WAV)
• default "other" track name
• bitrate and quality level when MP3 format is selected (see demucs docs)
• other demucs settings? (segment size, parallelism, CPU vs. GPU, clip mode, etc.)
• customize theme / waveform colors?

[will2dye4]

Fixed in v1.2.0.