do not re-establish an active sip connection multiple times

current state

• sip-connection is re-established every time a user clicks on an active language-channel
• this leads to audio-interruption (duration depending on webrtc handshake)

goal

• do not re-establish an active sip connection when clicking repeatedly on the active entry in the language menus
• provide uniterrupted audio when not actually changing the channel

implementation

• html5-logic only