PICARD-415 make lookup faster again

read_sparse is available starting with libdiscid 0.5.0.
The performance problem appears starting with 0.3.0 in general
and 0.3.1 for Linux.
Merge branch 'sparse_read' of https://github.com/JonnyJD/picard
Add a note about unrecommended libdiscid versions
PICARD-296: Cover file is saved repeatedly

If overwrite was enabled, the else suite was executed which saved the
cover file once for each track of an album even if there was only one
file. Check if the filesize is the same as that of the file we're trying
to save - if it is, don't save the file again.
Display version information for PyQt, Mutagen and libdiscid in about dialog.
Fixed exception in about dialog when libdiscid is not installed at all.

Signed-off-by: Philipp Wolfer <ph.wolfer@gmail.com>
Merge branch 'version-info' of https://github.com/phw/picard
Update NEWS.txt
Merge branch 'master' of https://github.com/brainz34/picard
Fix "vocal vocals" bug in performer: tag
Fix: Picard crashes if a plugin can not be loaded.

Signed-off-by: brainz34 <brainz34@musicbrainz.org>