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>

doc/ale.txt

@@ -646,6 +646,9 @@ problem will be displayed in a balloon instead of hover information.
 Hover information can be displayed in the preview window instead by setting
 |g:ale_hover_to_preview| to `1`.
 
+When using Neovim, if |g:ale_hover_to_floating_preview| or |g:ale_floating_preview|
+is set to 1, the hover information will show in a floating window.
+
 For Vim 8.1+ terminals, mouse hovering is disabled by default. Enabling
 |balloonexpr| commands in terminals can cause scrolling issues in terminals,
 so ALE will not attempt to show balloons unless |g:ale_set_balloons| is set to
@@ -954,6 +957,15 @@ g:ale_default_navigation                             *g:ale_default_navigation*
   buffer, such as for |ALEFindReferences|, or |ALEGoToDefinition|.
 
 
+g:ale_detail_to_floating_preview                            *g:ale_detail_to_floating_preview*
+                                                            *b:ale_detail_to_floating_preview*
+  Type: |Number|
+  Default: `0`
+
+  When this option is set to `1`, Neovim will use a floating window for
+  ALEDetail output.
+
+
 g:ale_disable_lsp                                           *g:ale_disable_lsp*
                                                             *b:ale_disable_lsp*
 
@@ -1177,6 +1189,16 @@ g:ale_fix_on_save_ignore                             *g:ale_fix_on_save_ignore*
   let g:ale_fix_on_save_ignore = [g:AddBar]
 <
 
+g:ale_floating_preview                                       *g:ale_floating_preview*
+
+  Type: |Number|
+  Default: `0`
+
+  When set to `1`, Neovim will use a floating window for ale's preview window.
+  This is equivalent to setting |g:ale_hover_to_floating_preview| and
+  |g:ale_detail_to_floating_preview| to `1`.
+
+
 g:ale_history_enabled                                   *g:ale_history_enabled*
 
   Type: |Number|
@@ -1235,6 +1257,14 @@ g:ale_hover_to_preview                                 *g:ale_hover_to_preview*
   instead of in balloons or the message line.
 
 
+g:ale_hover_to_floating_preview                       *g:ale_hover_to_floating_preview*
+                                                      *b:ale_hover_to_floating_preview*
+  Type: |Number|
+  Default: `0`
+
+  If set to `1`, Neovim will use floating windows for hover messages.
+
+
 g:ale_keep_list_window_open                       *g:ale_keep_list_window_open*
                                                   *b:ale_keep_list_window_open*
   Type: |Number|