Expansion Tile Fixes:

* Fix ExpansionTile description and preview rendering in the component library.
* Remove children padding setting.
* Fix visual density settings not applying right.
* Fix enabling/disabling expansion tile placeholders causing issues.
* Disable subtitle when dropping expansion tiles with no subtitles from the component library.

Author: SaadArdati

lib/src/api/models/variables_model.g.dart

@@ -121,8 +121,10 @@ class VisualDensityModel with EquatableMixin, SerializableMixin {
   VisualDensityModel copyWith({
     double? horizontal,
     double? vertical,
+    VisualDensityType? type,
   }) {
     return VisualDensityModel(
+      type: type ?? this.type,
       horizontal: horizontal ?? this.horizontal,
       vertical: vertical ?? this.vertical,
     );