Support XDG environment settings

[tikki]

The XDG Base Directory Specification is supported by many applications and aims to help a user keep their home directory a bit more tidy.

This PR adds support for the following locations from the spec:

• all cache files are put into $XDG_CACHE_HOME
• the config file lives in $XDG_CONFIG_HOME
• the database is stored in $XDG_DATA_HOME

To support existing users the code first checks if ~/.movielst exists and only uses the XDG dirs if ~/.movielst does not exist.

Some caches will have to be rebuilt though (or manually migrated), because their locations changed:

• ~/.movielst/cache -> ~/.movielst/cache/images
• ~/.movielst/dep -> ~/.movielst/cache/deps

The locations of the config.ini and movies.db are not affected (for legacy users).