Completion is not enabled on expression fields from custom widgets

[pcdavid]

The Slider widget introduced in #1914 defines 3 attributes typed InterpretedExpression.
There are normally rendered with a yellow background and have completion support enabled, but this is not the case for Slider.
This is because the test to detect these InterpretedExpression fields is too strict and assumes all widgets are defined inside the View EPackage itself.

Steps to reproduce:

1. Create a new FormDescription.
2. Add a Slider widget inside a Group.
3. Select the widget in the explorer.

In the Details panel, the 4 expression attributes should be rendered with a yellow background and support completion.