Make CapCollection a bit more useable

Collections are not stored in replapplication.objects anymore, but in
replapplication.collections. This fixes the IDs issue. There was no gain
from storing them in objects.

Completion right after cd should be faster (and should always have been
like that, it was pretty much a bug).

The display of do_ls() should be much clearer. Collections are always at
the end, have ids/title/backend like objects (but using a formatter
would be better than my current hack).

There are still many issues with the current implementation.
refs #774
closes #785

weboob/capabilities/collection.py

@@ -55,6 +55,7 @@ class Collection(object):
     id and title should be unicode.
     """
     children = Children()
+    backend = None
 
     def __init__(self, _id=None, title=None, children=None, fct=None):
         self.id = _id