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

test/test_hover.vader

@@ -25,6 +25,7 @@ Before:
   function! ale#floating_preview#Show(lines, ...) abort
     let g:floating_preview_show_called = 1
     let g:floated_lines = a:lines
+    return win_getid()
   endfunction
 
   function! ale#lsp_linter#StartLSP(buffer, linter, callback) abort