efl_ui_widget: fix disabled set behaviour
this fixes disabled set behaviour. This ensures that when setting
disabled twice, that unsetting it once does not break the overall state.
This never appeared in any real life example, because
elm_object_disabled_set is already checking for equalness. However, this
is not wanted here, because the simple setter can also be used to sync
the state with the parent, which appears to be helpfull.
Depends on D11550
Reviewed By: zmike
Differential Revision: https://phab.enlightenment.org/D11551