Use new translate_caa_type() to handle caa type translations
Merge Image() and CoverArtImage()

- Info dialog Artwork tab now displays types and comment for cover art images
- a tooltip gives info about the source (url, caa url, local file, tag, ...)
Make exit cleanup process more generic
Drop refcount/__del__ mess, make code much more reliable and simpler
Introduce image_info() to extract width, height from image data
Use real image files to test formats
Fix deprecated exception.message
imageinfo: add extension to result
imageinfo: simplify code and exceptions, image_info() -> identify()
Remove now unused LockableDefaultDict
Rely on imageinfo.identify to extract mimetype

- it does what mimetypes.get_from_data()
- .tiff is unused (afaik), so don't bother with it
- it is safer to analyze data for mime type (as data may not match provided mime type)
- when images are loaded from audio file tags, a check is done on valid format
Remove now unused util.mimetype
support_types=False is the default, unneeded
imageinfo.IdentifyError -> imageinfo.IdentificationError
imageinfo: differentiate each failure case

Add exceptions (subclassing IdentificationError):
- NotEnoughData
- UnrecognizedFormat

Add corresponding test code.
imageinfo: replace assert() with exception raise

Exception UnexpectedError may be raised if for some reason data parsing fails.
CoverArtImage.maintype -> @property
CoverArtImage.set_data(): fix up comment

No ref counter anymore.
Remove unused imports
imageinfo.identify(): improve description about exceptions