BasicPlasmoidHeading: Fix "configure" button's bindings

Mainly, binding on visibility was wrong for two reasons at once:

- Visibility is an implicitly propagated property, so what you read may
  not be what you wrote;
- Using other property as a guard sub-expression is not reliable due to
  unspecified order of signal propagation: dependent property may have
  not updated yet when its consumer already re-evaluates its binding
  using stale value as a guard.