Rearrange and dock tabs and panel groups in the editor workspace

[Keavon]

This should eventually hook up to the Rust editor backend to save the workspace layout but a frontend-only implementation is all this issue requires for now.

[Keavon]

~~Blocked on #225~~

[0HyperCube]

The main complexity with this is handling when multiple documents are in the screen at once. Currently we rely on active_document() to distribute messages in the backend and have one active tool for the entire editor. When backend responses are generated (i.e. a newly rendered svg), we then have no way to tell which document panel they are for.