cache: Add media_art_get_file()

This call is similar to media_art_get_path(), but using GFiles
for in/out path parameters. This older function now internally
uses the just added one, so internally operations happen on
GFiles.
extract: Add media_art_process_file()

This function is similar to the current media_art_process(), but
taking a GFile, mainly added for convenience in applications
already using GIO.
extract: remove unused function parameter
extract: Fix two compile warnings
extract: Include two missing headers

The call to media_art_plugin_init() was modified as it had the
wrong arguments.