Store character set preferences in GSettings
Store playlist mask preferences in GSettings
Remove local CDDB search function

It is very unlikely that people have a local copy of the CDDB, and the
settings code for this would be rather awkward.
Remove rename directory mask history

Use a single default mask instead.
Store default scanner masks in GSettings
Launch default audio player for directory

Use GAppInfo to discover the default player for the content type of the
first file in the list, and play the files in the directory using that
player.
Launch default audio player for selected files
Remove main configuration file handling

All user pereferences are now stored in GSettings. Mask lists and combo
box histories are still stored in files.
Fix some const string warnings

Found with -Wwrite-strings and -Wcast-qual.