Masking-focused feature requests for the Gradient tool

- [ ] Treat the Gradient Tool as a mask layer (currently no mask), allowing for solid color, gradient, or image fills.
- [ ] Automatically create a clipping mask for a single shape when filling it.
- [ ] Automatically add groups for selected shapes when filling multiple shapes and create group clipping masks (Is it a good approach to automatically create groups if layers are in different groups? Perhaps creating a clipping mask on the topmost layer and linking other layers to this clipping mask is a better method).
- [ ] Gradient Tool mask layers can select (fill area, fill stroke, and path).