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

Signed-off-by: brainz34 <brainz34@musicbrainz.org>
Fix tests for cover art vs formats, they couldn't fail.
Reduce cover art vs formats test code redundancy.
Fix test_ape failure due to incorrect APEValue type (auto)detection.

Force APEValue type on save to mutagen.apev2.BINARY, it is needed because
mutagen is trying to detect type, and set it to TEXT, and when it is tested
on load (apev2.py _load()), it doesn't detect cover tag because it isn't of
the BINARY type as expected.
Fix test_mp4 failure AssertionError: 'image/jpeg' != 'image/png'

value is already a MP4Cover, calling it again just set the format to default
JPEG, causing this issue on load.
Merge branch 'tests_formats_fix' of https://github.com/zas/picard
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.