[INTERNAL] add MUCNegotiationManager and PreferenceConstants

Change-Id: I1c092a15f735588d42e700cff1b000d1909a102d
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1653
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>
Tested-by: Jenkins CI