[GUI] Display how to draw a connection as a tooltip on a non-connected port
Drawing channel still seems to be difficult for somebody who did not
have a person teaching him/her.
It seems a natural thing is to hoover over a port (which can be added
simply from the palette), so bringing the information at that moment
makes sense.
(from redmine: issue id 2229, created on 2015-01-23, closed on 2018-02-01)