[vim] Popup window support for both Vim and Neovim

e.g.
  let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }

Based on the code from https://github.com/junegunn/fzf.vim/issues/821#issuecomment-581273191
by @lacygoill.

README-VIM.md

@@ -184,6 +184,7 @@ The following table summarizes the available options.
 | `dir`                      | string        | Working directory                                                     |
 | `up`/`down`/`left`/`right` | number/string | (Layout) Window position and size (e.g. `20`, `50%`)                  |
 | `window` (Vim 8 / Neovim)  | string        | (Layout) Command to open fzf window (e.g. `vertical aboveleft 30new`) |
+| `window` (Vim 8 / Neovim)  | dict          | (Layout) Popup window settings (e.g. `{'width': 0.9, 'height': 0.6}`) |
 
 `options` entry can be either a string or a list. For simple cases, string
 should suffice, but prefer to use list type to avoid escaping issues.
@@ -193,6 +194,16 @@ call fzf#run({'options': '--reverse --prompt "C:\\Program Files\\"'})
 call fzf#run({'options': ['--reverse', '--prompt', 'C:\Program Files\']})
 ```
 
+When `window` entry is a dictionary, fzf will start in a popup window. The
+following options are allowed:
+
+- Required:
+    - `width` [float]
+    - `height` [float]
+- Optional:
+    - `highlight` [string default `'Comment'`]: Highlight group for border
+    - `rounded` [boolean default `v:true`]: Use rounded border
+
 `fzf#wrap`
 ----------
 
@@ -276,44 +287,17 @@ The latest versions of Vim and Neovim include builtin terminal emulator
 - On Terminal Vim with a non-default layout
     - `call fzf#run({'left': '30%'})` or `let g:fzf_layout = {'left': '30%'}`
 
-#### Starting fzf in Neovim floating window
+#### Starting fzf in a popup window
 
 ```vim
-" Using floating windows of Neovim to start fzf
-if has('nvim')
-  function! FloatingFZF(width, height, border_highlight)
-    function! s:create_float(hl, opts)
-      let buf = nvim_create_buf(v:false, v:true)
-      let opts = extend({'relative': 'editor', 'style': 'minimal'}, a:opts)
-      let win = nvim_open_win(buf, v:true, opts)
-      call setwinvar(win, '&winhighlight', 'NormalFloat:'.a:hl)
-      call setwinvar(win, '&colorcolumn', '')
-      return buf
-    endfunction
-
-    " Size and position
-    let width = float2nr(&columns * a:width)
-    let height = float2nr(&lines * a:height)
-    let row = float2nr((&lines - height) / 2)
-    let col = float2nr((&columns - width) / 2)
-
-    " Border
-    let top = '╭' . repeat('─', width - 2) . '╮'
-    let mid = '│' . repeat(' ', width - 2) . '│'
-    let bot = '╰' . repeat('─', width - 2) . '╯'
-    let border = [top] + repeat([mid], height - 2) + [bot]
-
-    " Draw frame
-    let s:frame = s:create_float(a:border_highlight, {'row': row, 'col': col, 'width': width, 'height': height})
-    call nvim_buf_set_lines(s:frame, 0, -1, v:true, border)
-
-    " Draw viewport
-    call s:create_float('Normal', {'row': row + 1, 'col': col + 2, 'width': width - 4, 'height': height - 2})
-    autocmd BufWipeout <buffer> execute 'bwipeout' s:frame
-  endfunction
-
-  let g:fzf_layout = { 'window': 'call FloatingFZF(0.9, 0.6, "Comment")' }
-endif
+" Required:
+" - width [float]
+" - height [float]
+"
+" Optional:
+" - highlight [string default 'Comment']: Highlight group for border
+" - rounded [boolean default v:true]: Use rounded border
+let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }
 ```
 
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