Turn "dragui" into standalone package/library

[Inwerpsel]

This part of the codebase is almost ready to be used as a standalone library.

The name is not used in the code yet.

"dragui" seems a good fit, and I couldn't find other libs with the same name. If it would be taken, another option is "redragui".

What remains:

• Stable way to generate key
• Prevent panel config from needlessly changing (mainly: in some cases if an element ends in the same position, its order index is incremented anyway)
• Improve naming of some primitives (DispatchedElement, PanelMap, MovablePanels, ...)
• Add the right sprinkles of TypeScript
• Try get rid of React context (or determine it's always harmless)
• Figure out and set up useful library config (package, basic internals)
• Add missing parts, correct outdated in documentation