This PR updates function signatures, docstrings, and gpui's other
documentation to reflect it's new state following the merge of `Model`
and `View` into `Entity` as well as the removal of `WindowContext`.
Release Notes:
- N/A
Fix bugs caused by the window context PR, where the window could be on
the stack and is then requested from the App.
This PR also adds derive macros for `AppContext` and `VisualContext` so
that it's easy to define further contexts in API code, such as
`editor::BlockContext`.
Release Notes:
- N/A