WIP: Refactor out ElementContext from WindowContext (#4166)

During the doc parties, I've sometimes struggled with how and where to
describe the high level intention behind various APIs that are available
in GPUI. Fortunately, rust doc has a tool for this: Module level
documentation. However, writing this kind of documentation for the
`WindowContext` is difficult, as it bundles up the nice, high level GPUI
APIs for entity management and view rendering, with all of the low level
imperative APIs that `Element`s use to draw to the screen. Splitting
these apart into an `ElementContext`, which contains a `WindowContext`,
gives us a way to corral the element systems into their own API and
module, allowing us to document the Element system on it's own terms,
and enforce the correct usage of our APIs, and helps people who are
exploring our APIs through autocomplete to not be overloaded with
non-applicable methods.

Release Notes:

- N/A