Problem:    Unable to customize the column next to a window ('gutter').
Solution:   Add 'statuscolumn' option that follows the 'statusline' syntax,
	    allowing to customize the status column. Also supporting the %@
	    click execute function label. Adds new items @C and @s which
	    will print the fold and sign columns. Line numbers and signs
	    can be clicked, highlighted, aligned, transformed, margined etc.

Problem:    Handling 'statusline' errors is spread out.
Solution:   Pass the option name to the lower levels so the option can be
            reset there when an error is encountered. (Luuk van Baal,
            closes vim/vim#11467)

https://github.com/vim/vim/commit/7b224fdf4a29f115567d4fc8629c1cef92d8444a

problem: code for drawing statusline is arbitrarily spreadout between drawscreen.c, screen.c and buffer.c
solution: move it to a new file statusline.c

- rename archaic internal name "status match" to public name "wildmenu"
- showruler() does not show the ruler. it show anything which displays
  info about the cursor. Rename it accordingy.

Problem:    Get E685 and E931 if buffer reload is interrupted.
Solution:   Do not abort deleting a dummy buffer. (closes vim/vim#5361)
https://github.com/vim/vim/commit/a6e8f888e7fc31b8ab7233509254fb2e2fe4089f

This includes a partial port of Vim patch 8.2.2569 and some changes to
nvim_eval_statusline() to allow a multibyte fillchar. Literally every
line of C code touched by that patch has been refactored in Nvim, and
that patch contains some irrelevant foldcolumn tests I'm not sure how to
port (as Nvim's foldcolumn behavior has diverged from Vim's).

Problem:    Number of status line items is limited to 80.
Solution:   Dynamically allocate the arrays. (Rom Grk, closes vim/vim#7181)
https://github.com/vim/vim/commit/8133cc6bf454eb90bb0868f7cf806fce5c0c9fe6

The members of stl_item_T have not been prefixed with stl_ contrary to
the vim patch because the amount of stl_ prefixes on single lines of
code in that region was hurtful to readability.

Improve coverage of `build_stl_str_hl`.
Minor removal of dead code in the tested function.