gc_contact.jid and gc_contact.resource are empty in anonymous rooms. In non-anonymous rooms those contain the real jid and resource of the contact.
We must not mix those information with the jid of the contact in the groupchat: We assumed that "test@conference.gajim.org/nick" was the Jid and 'someresource' the resource. So we tried to talk to 'test@conference.gajim.org/nick/someresource'.

re-enable the ability to connect as invisible: check that we don't send presence with show=invisible at the correct place, after we diferenciate invisible conection from not-invisible connection. Fixes #5473

update README file to depend on python-dbus >=0.81, which is needed for bus.list_names() function. Fixes #5474

Fix bug that prevented avatars to be shown in groupchats. Bug was due to a wrong re-use of a local variable (local variabes are evil :-).

Additionally: Remove some appreviations and unneeded local variables.

don't show in roster / notification that a contact unsubscribed us if we already popup up the dialog. Fixes #5470

I remove this line because it makes no sense to assume that we have joind a groupchat, just because someone request a VCard for a jid which pretends to be from a Groupchat.

 * set default file type for avatars centrally
 * remove client side decision logic determing if a avatar loookup shall happen for a groupchat contact or for a normal contact
 * remove unused method (this one was triggering the traceback in #5461)