prevent traceback when changing resource of a control when there is no event for the account at all. Fixes #5375

merge roster_contact_context_menu.glade, chat_control_popup_menu.glade and zeroconf_contact_context_menu.glade

This commit does not improve overall structure in gajim in a logical way.
It seems untested (is broken) and is therefore rejected.

Resource conflict dialog has now a timeout. Fixes #4590

This kind of work will come after 0.13

This is the first commit of a long serious of cleanup commits.
Simple testcase included to test the slightly enhanced Interface dispatcher.