"Disabled" widgets

A useful class for widgets are conditional, and "enabled" only when certain Parameter conditions are met.
Typically, interacting with them will force one of these conditions too.
Right now, the widget is forced to listen to these "enabling" parameters.

Is there something more composable? So that the same widget could work with a Parameter that can be disabled by another Parameter, or just vanilla and never be disabled.

Should the disabled state be in Parameter?
Should the disabled flag be in UIElement?

Start by tracing how widgets currently listen to enabling Parameters, then compare the roles of Parameter and UIElement in the existing widget code. Decide where a composable disabled state belongs and define how the same widget should work with a conditionally disabled Parameter or with no disabling behavior; the issue provides no file or test names.

Written by the indexing model from the issue text.