feat: reorder buffers with mouse drag

[Iron-E]

Closes #261

The feature is automatically enabled when clickable = true during setup.

Todo:

• [ ] gif on readme?
• [x] check on_key isn't required (it was)

[Iron-E]

Not enough time to review & test but go ahead when you think it's ready.

What is your opinion on caching layout changes to make buffer dragging faster? It'd be easy to revert that part of this PR if you think it should be.

Pros:

• Faster buffer dragging
• Aligned aporoach for layout.buffers and other results (previously only one result was cached, now every result is)
• Easier to extend barbar in ways that need an understanding of layout

Cons:

• More side effects of calculate (it's a very impure function now)
• Initial performance gain probably marginal, dragging performance was already acceptable

[romgrk]

I think simplicity first, and only do the performance optimization if it's required.