whenXXXDo: convenience methods should be removed

[plantec]

because they introduce limitation for elements event handlers management.
example:

but := ToButton new.
but whenClickedDo: [:event | bla bla ].

can be coded as :

but := ToButton new.
handler := BlEventHandler on: BlClickedEvent do: [:event | bla bla ].
but addEventHandler: handler.

which allows to remove/replace the event handler as in:

but removeEventHandler: handler.
handler := ...
but addEventHandler: handler.

[plantec]

related to #44

[tinchodias]

Also related to pharo-graphics/Bloc#257