Suggested pencils down time

Last week I worked a little with XEP describing my protocol and then I focused on changes changes proposed by Remko. I changed texts informing about session status, added button to cancel whiteboard request, added "Opened whiteboards" section to chats tab(which allows to see opened whiteboard chats and bring it up), added handling of sessions where one side goes offline, reorganized a little session handling code, "ifdefed" whiteboard code to include it only if experimental flag is set, finally added eraser icon and modified UI a little. At the end of the week I rebased my branch on master and did some final touches according to Remko's comments. He said that he suspects that whiteboard can be integrated quite soon after GSoC pencils down under experimental.