the widget to describe.
the values to overlay over the ambient context, or undefined if this contribution does not
describe the given widget. A contribution owning a key should return an explicit "unset" value for the
widgets the key does not apply to, so that the ambient value of another widget cannot leak in.
Contributes the context key values that describe a particular widget, so that
whenclauses about that widget - such as the ones of its tab bar toolbar items - can be evaluated without consulting the ambient context, which describes whichever widget currently has focus.