Please add colors for WinBar (NeoVIM 0.8 new feature)

[alexeygumirov]

WinBar highlight group is needed.

Without WinBar highlight group, the WinBar uses standard background and does not stand out.

I made my own modification in the NVIM LSP section for WinBar,

  " Neovim (LSP) diagnostics
  if has('nvim')
    ...
    exec 'hi WinBar' . s:fg_aqua . s:bg_tabline_bg

With it WinBar looks a bit better:

But this is just a temporary hack and I am not fully satisfied with the colors.
I think this feature requires a bit better color taste than mine and better color design.