[FIX] #570 Senseless option in Roster when no projects in workspace 

Put the correct logic to display this entry into the class that creates
the menu instead of creating this entry via XML in the plugin.xml file.

Change-Id: I27254ab579dda2bf51eb521870bd147d4303a5d7
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1677
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>