Fix: Picard crashes if a plugin can not be loaded.

Signed-off-by: brainz34 <brainz34@musicbrainz.org>
Make config and log importable

Conflicts:

	picard/ui/mainwindow.py
Fix tests
CAA: Correctly deal with images with no type set

Those have an empty "types" list - we need to add a type because
_caa_append_image_to_try_list expects one to be there.
Regenerate picard.pot

New strings include those for collection management and cover art types.
Merge branch 'master' of https://github.com/brainz34/picard into mb-master
Merge branch 'importable-config-log' of https://github.com/mwiencek/picard into mb-master

Conflicts:
	picard/plugin.py
Do not rely on Pythons conversion from int to bool when connecting slots

The stateChanged signal of QCheckBox emits an int but the setEnabled slots
expect a bool.
Use picard.util.partial instead of functools.partial
PICARD-417: Re-add the signals of rename_files

Those got lost in efd5277
Add a comment explaining the fix for PICARD-417
Fix exception caused by 3f6c64d

If metadata.length is 0, it's Falsy and not added to the tagdiff stuff. This
stores it as a str because it's simpler than adding `or value != 0`
everywhere.