Enable shadowing on grouped shapes

[tpvdl]

**Problem Statement:**

Currently, Draw.io does not allow shadows to be applied to a group of shapes in a way that treats the group as a single entity. Instead, shadows are applied individually to each shape within the group. This behavior can result in overlapping or obscured shadows, creating a visually inconsistent and inaccurate representation, especially for complex designs.

**Proposed Solution:**

Introduce functionality that allows shadows to be applied to a grouped set of shapes as a unified entity. This means the shadow effect would be calculated as if the group were a single combined shape, eliminating overlapping and inconsistent layering issues.