element: Make return value of setters mean something

element: Make return value of setters mean something

Setters return values should return %FALSE **only** when the value
could not be set, not when unchanged or when the subclass handled
it itself!

This patches makes it so the return value is meaningul by allowing
subclasses return anything different than `TRUE` or `FALSE` (convention
is -1) to let the subclass now that it took care of everything and
no signal should be emited.