Add a BoxShadow element

This intends to provide a configurable rectangular "drop shadow". The
API is modeled after CSS/HTML5 Canvas where the element can be "bound"
to an existing rectangular shape (geometry and radius), the offset can
be used to place the shadow and color and blur configure the shadow.

The shadow's color fades into transparent.

TODO (in subsequent changes):
 * Documentation
 * Qt implementation

sixtyfps_runtime/interpreter/dynamic_component.rs

@@ -489,6 +489,7 @@ fn generate_component<'id>(
                 rtti_for::<Window>(),
                 rtti_for::<TextInput>(),
                 rtti_for::<Clip>(),
+                rtti_for::<BoxShadow>(),
             ]
             .iter()
             .cloned(),