Return floating_preview window id (#4828)

Make ale#floating_preview#Show more similar to popup_create and return
the id of the window so it's easy to set the filetype of the resulting
buffer.

Update test stub version of Show() to return a win id (the current window
since it's not actually creating a window).

Test
* both tests still pass

autoload/ale/floating_preview.vim

@@ -21,6 +21,8 @@ function! ale#floating_preview#Show(lines, ...) abort
     else
         call s:VimShow(a:lines, l:options)
     endif
+
+    return w:preview.id
 endfunction
 
 function! s:NvimShow(lines, options) abort