main/path: Inline constants used once
spotify: Rename 'cache_path' to 'cache_dir'
local: Rename 'music_path' to 'music_dir', 'playlist_path' to 'playlists_dir'
mpd: Rename 'music_path' to 'music_dir'
Replace 'folder' with 'dir'
path: Use our expand_path instead of os.path.expanduser
Merge pull request #403 from jodal/feature/rename-path-to-dir

Use "dir" instead of "folder" or "path"
docs: Add 'confval' object type to Sphinx
docs: Fix typo
docs: Use :confval: to make config docs linkable
docs: Check for 'MIXER_TRACK_' to not confuse with old 'MIXER_TRACK' setting
docs: Fix typo
docs: Update config page howtos