Apply the (static) styles defined for widgets to their "preview" in the Form Description Editor

[pcdavid]

When configuring forms using the Form Description Editor, the widgets visible in the editor all have the default styles. For the WidgetDescriptionStyle attributes which are static (not computed expressions, not in conditional styles which depend on dynamic expression evaluation), the widgets' preview should show directly the customized style value (e.f. foreground and background colors, text style, etc.).

[pcdavid]

FormDescriptionEditorComponent creates instances of the same FormDescriptionEditorWidgetComponent type for all types of widgets, but the actual style properties are different for each one.

I see two options:

1. we pass some generic structure with all the style properties, determined reflectively/dynamicaly, and re-interpret this generic "style map" on the front in each widget (e.g. sirius-components-formdescriptioneditors/src/LabelWidget.tsx) to display them.
2. we create specialized types of components/props (on the back) and widgets/widget props (on the front).

The second option would be more type-safe, but involves a lot of additional, very boring and mechanical code.

[pcdavid]

The DRAFT PR above follows the first approach. Note that:

• it only handles the Textfield widget;
• it relies on reflective EMF code to discover all the style attributes;
• it still requires some duplication between TextfieldPropertySection.tsx and TextfieldWidget regarding the computation of the CSS class to use for the underlying MUI Input.