Add context menu support

This patch adds support for adding context menu entries
to connected users. As these callbacks are - in contrast
to all others in mumo - user specific they do not fit
well with the existing mumo architecture. To cope with
this creating these callbacks is now handled directly
inside the manager on behalf of the remote manager.
The remote manager then handles forwarding any context
menu actions to a user selected handler.

This patch also removes the vestigial parts of context
menu handling already present but unused in mumo as they
were an architectural dead end.