i3,sway: add bar color options for the focused output

[sigprof]

Description

Both i3bar and swaybar can use different colors for the bar on the currently focused monitor output; add color options for this feature.

It is not possible to add these options using xsession.windowManager.i3.config.bars.*.extraConfig while using other bar color options at the same time, because all color options must be inside the colors { ... } block, so the only way to use extraConfig is to move the whole bar color configuration there, losing the convenience of existing options.

New tests added for both i3 and sway, and the expected output for existing tests is adjusted (changes are limited to extra blank lines; avoiding the changes completely would result in much less readable code in modules/services/window-managers/i3-sway/lib/functions.nix, which would also be significantly different from the code for existing options).

New options are not defined using mkNullableOption, like the existing options are, because mkNullableOption sets the default value to non-null for state versions < 20.09, and this would break old configs, because these default values would override normal colors that might be configured, so the null default is really required.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

[sumnerevans]

LGTM. Thanks!