Add GSettings schema

Leave the keys that depend on enums and flags commented for now, as the
enum values need to be generated with glib-mkenums.
Use GSettings for most application settings
Generate enum descriptions with glib-mkenums
Generate GSettings enum descriptions
Store the file list sorting mode in GSettings
Store the scanner window mode in GSettings
Store the file extension case mode in GSettings
Store the playlist content mode in GSettings
Store ID3v1 and ID3v2 iconv options in GSettings
Store rename filename encoding in GSettings
Store the process fields option in GSettings