This PR:
- Fixes several bugs with dock resizing
- Adds a new drag move API for resizes 
- Adds special debug styling for elements, including auto-opening Zed at
the element's location in source
- Changes the `cx.paint_quad()` API to take a type and adds several
helpers to create this type.
- Adds `()` as the empty element, and uses it to implement a derive
macro for the `Render` trait.

Release Notes:

- N/A