cache: Added media_art_remove_async() and _finish()

Part of this API changes makes the media_art_get_{path|file}() APIs not do
i/o operations like creating the cache directory. This is now done in
media_art_process_new().

https://bugzilla.gnome.org/show_bug.cgi?id=724879
cache: Fixed removing media art, was failing badly
extract: Add async APIs for all functions

https://bugzilla.gnome.org/show_bug.cgi?id=724879
Merge branch 'async-api'