Refactor front end to be consumed as an extensible library

[lukehb]

The current front end (the loose collection of html, js, and css) does not serve its purpose as a reference because:

1. It is quite complex and all crammed into a few files.
2. There are no obvious extension points in the code.
3. When upstream changes are made users who have modified the code are left manually incorporating fixes.

The net result is most users end up with a modified version of this messy frontend and are applying fixes as they come in from UE releases.

I propose we move the front end to a lib-psfrontend that can actually be consumed as library with sensible extension points and API surface so that users can bring it in as library and make whatever custom extensions are suitable for their application.

[lukehb]

As 61e8273 this is now complete!

More refactors will come later to separate out the UI from the core library, but that will be its own issue.