Python's enums are not extensible, values can't be added and they can't
be subclassed. This is problematic if weboob devel adds new values,
because stable weboob will not be able to use them. They would need to
be replaced by a default value or None, which loses information.

Instead, implement custom enums that contain plain constants and support
adding values aftewards.

Since ItemElement calls constructors with no parameters, it was
impossible to use with BaseImage since a parameter was mandatory.

__str__ is reimplemented in terms of __unicode__ and __repr__ else.

Signed-off-by: Simon Lipp <laiquo@hwold.net>

Alexandre Morignot authored Dec 05, 2014 and Romain Bignon committed Dec 13, 2014

The modules using this capability and galleroob have been corrected too

Used in galleroob and present in all CapGallery modules

There is now a "latest" collection (or "latest_nsfw").
The feature didn't look much used, since it didn't work on many
backends.
Using collections will make it easy to support other things
like most viewed, featured, etc.
As a bonus, I added tests for every backend with the feature.

It's a huge copy-paste of CapVideo, and will have to be factorized ASAP,
though it's going to require more work and thinking, and maybe CapPath.