Neovim-qt: Add global sentinel variable to identify neovim-qt

[legendre6891]

The idea is to introduce a variable (say, g:NvimQt) such that g:NvimQt == 1 when neovim is launched with nvim-qt. This allows for nvim-qt specific customization.

See this comment.

[justinmk]

Nvim 0.3.x has nvim_get_chan_info(). When a UI connects it can call nvim_set_client_info() to set the "client" field of nvim_get_chan_info().

It would be strongly preferred for Nvim-Qt to do this instead of adding another special-case variable. This gives users a common mechanism for discovering info about connects clients/UIs.

[equalsraf]

Set channel info in #458. I also added a GuiName function you can use in ginit.vim. Try something like this.

if GuiName() == 'nvim-qt'
    echom "Oh no ..."
endif

[equalsraf]

Closing this - it is now available with the latest version.