some error messages about invalid arguments

untill there is a stable 2.8 version

causes a TB when gajim is started from
noninterractive shels like idle (fixes #1506)

default with for single msg window is somehow more sane now (so we can see subject easier). fixes #1505

when Drag And Drop from a group to another, popup a context menu to ask if we want to change group or make as subcontact

Removed threads and mutexes in connection.py
and gajim.py.
Add resolver through nslookup command, removed
dnspython and pydns (for now).

and it sucks losing a well thought out message.

introducing history_manager.py; button do not work and yes I plan to allow multiple selection of messages