XmppClient cleanup.
Allow XmppClient to use arbitrary property names in GUIMessages.
Remove duplication by calling CreateGUIMessage when receiving chat messages.
Inline PushGUINotification.
Use std::string in the GUIMessage because every data source is of that type.
Apply the wstring_from_utf8 conversion to ensure correct display of less common UTF characters instead in the GuiPollMessage method.
Add room subject change chat message.
Differential Revision: https://code.wildfiregames.com/D835
Refs #4482
Comments by fpre, Vladislav and leper