tests: Removed unused function
extract: Make GFile first parameter for _process_file()
extract: Allow error reporting with API using GError
extract: Refactor media_art_process_file() to simplify the function
extract: rename (private) media_art_heurstic and make args consistent with others
tests: Test for no error with media_art_process_file() with real use case
extract: Switch to initable MediaArtProcess object  from _init/_shutdown()
storage: Make g_message() calls g_debug() calls, output is unnecessary
extract: Use a more logical ordered heuristic for caching strings
extract: Add GError * reporting right down to the plugins (pixbuf/qt)
tests: Don't expect media art extraction for NULL data buffer for !images
extract: Add GError for _NO_TITLE, which is required in some cases
extract: Add _SYMLINK_FAILED error and report symlink() failure in get_heuristic()
build: Improve warnings used when compiling

Use CC_CHECK_FLAGS_APPEND m4 macro to make sure GCC supports flags

This is after finding out that some platforms or versions of GCC don't
support flags like -Wmaybe-uninitialized

The attributes.m4 is stolen from systemd who stole it from xine-lib.
docs: Fixed Makefile so we don't fail on distcheck with --enable-gtk-doc

Use $top_srcdir, not relative links
libmediaart: Split API into  _process_buffer() and _process_file()

This is quite an API break, but the previous API was confusing and a monster,
so this was really necessary.

The unit tests have been put in place to make sure things work correctly too.
extract: Fixed 'since' versions in documentation
Merge branch 'api-cleanup'