patch 9.1.1817: popup: there are some position logic bugs

patch 9.1.1817: popup: there are some position logic bugs

Problem:  popup: there are some position logic bugs
Solution: Refactor position logic and fix a few bugs
          (Girish Palya).

This change does the following:

- Simplified and rewrote horizontal positioning logic (was overly
  complex).
- Split horizontal and vertical positioning into separate functions.
- Fixed missing truncation marker (e.g. `>`) when items were truncated
  and `pummaxwidth` was not set.
- Fixed occasional extra space being added to menu items.
- Update tests

closes: #18441

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>