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
When "save_only_front_images_to_tags" is set, save only 1 front image.

Introduce helpers to clarify and fix code:
- is_front_image()
- save_this_image_to_tags()
- image_type_from_id3_num()
- image_type_as_id3_num()

Fixes:
- honor "save_only_front_images_to_tags" option for apev2 format too
- save only caa front image to tags when "save_only_front_images_to_tags" is set
  and image source is CAA, use 'front' flag from json.
  Other sources should have only 1 front image anyways.