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>

plugin/ale.vim

@@ -138,6 +138,15 @@ let g:ale_set_balloons = get(g:, 'ale_set_balloons', has('balloon_eval') && has(
 " Use preview window for hover messages.
 let g:ale_hover_to_preview = get(g:, 'ale_hover_to_preview', 0)
 
+" Float preview windows in Neovim
+let g:ale_floating_preview = get(g:, 'ale_floating_preview', 0)
+
+" Hovers use floating windows in Neovim
+let g:ale_hover_to_floating_preview = get(g:, 'ale_hover_to_floating_preview', 0)
+
+" Detail uses floating windows in Neovim
+let g:ale_detail_to_floating_preview = get(g:, 'ale_detail_to_floating_preview', 0)
+
 " This flag can be set to 0 to disable warnings for trailing whitespace
 let g:ale_warn_about_trailing_whitespace = get(g:, 'ale_warn_about_trailing_whitespace', 1)
 " This flag can be set to 0 to disable warnings for trailing blank lines