Ref: #9342
Adds the option to have a single global statusline for the current window at the bottom of the screen instead of a statusline at the bottom of every window. Enabled by setting `laststatus = 3`.
Due to the fact that statuslines at the bottom of windows are removed when global statusline is enabled, horizontal separators are used instead to separate horizontal splits. The horizontal separator character is configurable through the`horiz` item in `'fillchars'`. Separator connector characters are also used to connect the horizontal and vertical separators together, which are also configurable through the `horizup`, `horizdown`, `vertleft`, `vertright` and `verthoriz` items in `fillchars`.
The window separators are highlighted using the `WinSeparator` highlight group, which supersedes `VertSplit` and is linked to `VertSplit` by default in order to maintain backwards compatibility.
PR #8221 took a short-cut when implementing the tests: screen.lua would
translate the linegrid highlight ids back into the old per-cell
attribute description.
Apart from cleaning up technical debt, this enables to check both rgb
and cterm colors in the same expect(), which previously was needlessly
restricted to ext_hlstate tests only.
