Adds tagging support for non-taggable file formats. The tags can be stored in SQLite database or written as external files to the same directory as the target audio files.

For the component to work properly in foobar2000 v1.4 the "External Tags (reader)" decoder must have higher priority than the format to be tagged.
The "External Tags (writer)" decoder should have lowest priority so external tags won't take priority over native tags of any format.

On foobar2000 v1.5 the "External Tags (reader)" decoder is not used and won't be visible. The "External Tags (writer)" decoder is only needed for context commands.

• Fixed fallback album art writer to only create external tag when the art isn't blank.
• Fixed album art fallback writer to create external tag for all subsongs of the file and not just hardcoded subsong 0.