local: Create empty tag_cache if it doesn't exist
local: Log how many tracks and playlists are loaded
spotify: Use 'playlists' instead of 'playlist(s)' for consistency
main: Show 'Disabled extensions: none' instead of nothing
main: Log what backends/frontends are started
mpris: Log who connected to D-Bus
scrobbler: Log who connected to Last.fm
main: Use 'none' instead of emptry string

Same logic as for extensions are now applied to backends and frontends.
path: Test get_or_create_file()
path: Close create file right away
path: Let get_or_create_file() create missing dirs
main: Remove redundant dir creation
local: Use path helpers to create tag cache
local: Move data files into an extension specific dir
local: Create all needed dirs/files on startup
Merge pull request #404 from jodal/feature/better-startup-messages

Better startup messages