Add configuration to config file for NVIM_GTK_NO_HEADERBAR=1

[sdroege]

Having to set an environment variable seems strange if there is already support for a configuration file.

If you have any suggestions for the name of the configuration I can create a PR that adds this.

[Lyude]

We could add a GUI method for this for the time being, but I'm hoping to start using things like guioptions in the future wherever possible - and g:NvimGtk_* style variables for everything else. This sounds like something that would be a candidate for a g:NvimGtk_* style variable since neovim itself doesn't really have any concept of a header bar.

That might be easy enough to implement right now? I'm not sure, but the basic idea would be:

• Set a default value with neovim to the global config variable (e.g. g:NvimGtk_* before we call nvim.ui_attach(…))
• Monitor said variable using an autocmd (Subscription from src/subscriptions.rs can be used for this), also try to do this before nvim.ui_attach()

This will allow the user to set it in either init.vim or ginit.vim.

(also, apologies ahead of time: some of the initial UI init code is kind of a mess, since there's some parts that are done async via plain threads which were written back when daa84 maintained this. They're only messy now since I've started using proper rust async code elsewhere, but I haven't taken the time to clean this up)