* implement vim popups for preview
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
* floating window: document that vim supports it
* lint: fix indent/cont. lines
530b38de34