Improve CUI support

See explanation here:
http://www.minecraftforum.net/topic/2171206-172-worldeditcui/page__st__60#entry28552026

Basically this allows the handshake packet to overwrite the protocol version of WECUI (when a player joins with another client before the session expires) and also sends the current selection to the client when a handshake is received (the selection is not neccessarily empty when a handshake is received). This also allows WECUI to refresh the current selection by sending a handshake packet. That is needed for BungeeCoord servers. Currently this functionality is achieved in the mod by sending "/we cui" on a world change.

Additionally removed unneccessary SelectionShapeEvents, as they are alwys sent before describeCUI gets called in the LocalSession.