Added a white board

Added a white board activable by the root user. The board is
synchronized with every connected user. Uses the canvas tag.
Minor style changes

For white board, :
- Background color : white
- Delete style not implemented
action after exit white board

If the animator leave the white board.
Slaves are notify.
action after exit white board by slaves

If a slave quit the white board , Upon reopening the root's white board
is reload.
Merge remote-tracking branch 'origin/dev' into node_update