Problem: cannot style popup window (border, shadow, etc)
Solution: Extend the 'completepopup' option with additional properties
(Girish Palya)
This patch extends the 'completepopup' option with additional settings
to allow more configuration of info popup window.
New values:
```
- close "on" (default) or "off"
- resize "on" (default) or "off"
- borderchars specify eight characters (separated by semicolons) to
draw the popup border: top, right, bottom, left,
topleft, topright, botright, botleft.
- borderhighlight highlight group for the popup border characters
- shadow pum shadow
```
closes: #18487
Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
1a09f11f5d
