Fix memory leak in C++ GIOStream wrapper
Fix an invalid read when reading MP4 cover art
Clear empty fields in MP4 tags

The existing tag is loaded from the file and modified before being
written out. For "extra" fields including cover art and album artist,
the existing tag field was preserved in the case that the corresponding
tag field had been cleared from the UI. Fix this be explicitly removing
the cleared tag field.
WIP Use GIO when reading and writing FLAC tags

TODO: Handle non-local files with a memory- (or file-) backed IOStream.
Simplify differences between reading and writing callbacks. Verify that
errors are caught at the first problem and returned to the caller.