Engine: re-apply MessageBuilderSettings on config changes

If highlight words were changed, the old words were still highlighted and not
the new ones.
Engine-XMPP: refactored CreateMessage to get rid of multiple boolean parameters
Engine-XMPP: fixed hilighting of /joinas nickname (closes: #958)
Engine-XMPP: when joining a room-jid that is not actually a room-jid a nullptr exception could occur
Merge pull request #190 from oli-obk/fix/joinas_hilight
Merge pull request #191 from oli-obk/fix/joinas_nullptr
Merge branch 'stable'