- Adds a new `ObjcProperty` type, which wraps our Objective-C objects in
a Rust-backed Rc/Refcell combo. This in general makes understanding
reference counting/storage more digestable when reading through the
codebase and leans more on Rust to do its thing than the Objective-C
runtime.
- Most widgets that need to implement `Layout` now only need to provide
a slot for running a handler with the underlying node.
- Further documentation work ongoing.
