[REFACTOR] moved MUC classes

Moved MUCStateManager and MUCSessionPreferences into the muc base
package. Renamed MucSessionPreferences to MultiUserChatPreferences.
Renamed MUCStateManager to MultiUserChatStateManager and changed the
visibility to package private.

Change-Id: Idc7b5f6a8d2ec03e477d187c5b80aba5e8651ea0
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1440
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>