Fixes a couple of NPEs
Make sure that video content is added, if necessary, after a peer is put off hold.
Fixes initialization of the 'hold' menu item.
Only allow contacts from the same account as the videobridge to be invited into
videobridge conferences
Fix the 'senders' set in content-add in case the device doesn't allow sending.
Moves the 'senders' field from CallPeerMediaHandlerJabberImpl to
CallPeerJabberImpl. Minor fixes to comments, logging and sending error IQs.