Details on implementation
-------------------------
- we make use of the |popupwin| api
- we split implementations (Nvim* vs. Vim* prefix) and call the right
  one based on has('nvim')
- we follow a similar structure in each function, using the relevant API
  - popup_list, win_execute, popup_settext in VimShow
  - popup_create in VimCreate
  - popup_close in VimClose

Some differences
----------------
- we DON'T have VimPrepareWindowContent because we use arguments to
  popup_create for borders, padding, etc., and it also takes care of
  buffer creation.
- we follow the protocol of setting and using w:preview for information,
  but we only need the ID
- InsertEnter is the only autocommand required, because of
  popup_create's moved argument. Any cursor movement with 'any' will
  close the popup. This in turns means VimClose is only called from
  InsertMode, so no mode-restoration necessary
- we don't tweak too much in the buffer because vim's popup buffers
  already have most relevant settings and aren't editable without
  calling popup functions.
- I enabled scrollbars, close buttons, dragging, and resizing
- vim popups get as big as they need to by default, so no worrying about
  truncating/hiding/size

Note: we might want to consider changing w:preview to w:ale_preview to
avoid clashes if someone else tries to use the same variable