Add nvim floating window support (replaces #3314) (#3470)

* Add nvim floating window hover support
* Add configuration for float to replace preview
* preview#ShowFloating: qualify local variables
* Configure floating preview usecases individually

Also:
  * Extract floating preview to its own file.
  * Ignore 'stay_here' option. Moving into the floating preview window
    seems confusing at best.
  * Re-use existing floating preview window if it's still up.
  * Flush out floating preview documentation.

* Watch cursor position changes per window

Floating previews open a new window, so when that window is written to,
it moves briefly there at a different position than the original window.
This makes repeated positions detected when positions are tracked at a
s: level. Instead, we change the variable to window scoped, which only
fires a message if the cursor has changed from the last position in
*that window*.

* g:ale_floating_preview cleanup
* floating_preview: add ALEDetail tests
* Fix fecs test missing runtime call
* Add ALEHover floating preview tests

Co-authored-by: Jan-Grimo Sobez <jan-grimo.sobez@phys.chem.ethz.ch>

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