Proposed changes editor features (#18373)

This PR adds some more functionality to the Proposed Changes Editor
view, which we'll be using in
https://github.com/zed-industries/zed/pull/18240 for allowing the
assistant to propose changes to a set of buffers.

* Add an `Apply All` button, and fully implement applying of changes to
the base buffer
* Make the proposed changes editor searchable
* Fix a bug in branch buffers' diff state management

Release Notes:

- N/A