Implement separate *NC control highlight groups

[HiPhish]

This is the continuation of #174, it fixes the remaining issue I mentioned over there. Now the background should work regardless of which window carries the control UI. I wish Vim's highlight system had a sort of "inheritance" feature that is between link and defining from scratch. Then we would not have to do this ugly highlight group patching.

For each of the control highlight groups there is now a corresponding highlight group with the same name plus NC as suffix. When entering or exiting the window which carries the support groups we reset the 'winbar' option such that the highlight groups are either normal or *NC.

The reason for this is that the user might have a different background for the highlight groups WinBar and WinBarNC. Whenever the user exits or enters a window, the editor will flip between these two groups, so we have to flip our custom highlight groups accordingly as well.

[HiPhish]

I have fixed the merge conflicts. A word of note: you correctly found that vim.api.nvim_get_hl_by_name only exists in Neovim 0.8+, but the ability to pass a table to vim.cmd only exists in 0.8+ as well. I put an if has("nvim-0.8") block around the entire patching thing so none of it will be run on older versions of Neovim. It will still look ugly like before, but it will work.

We could glue together strings for vim.cmd if you prefer that, but I think it's better not accumulate technical debt. If we decide to abandon support for older version of Neovim we just have to remove a few if checks instead of rewriting a bunch of vim.cmd arguments.

What is you opinion?

[rcarriga]

I'm OK leaving it as is, this module is pretty stable and I'm not planning to maintain older versions for much longer so it'll be cleaned up anyway 😄 Thanks for the followup!