Add neovim-qt compatible layer (#5)

Add shim to rtp, add gtk gui flag

runtime/plugin/nvim_gui_shim.vim

@@ -0,0 +1,45 @@
+" A Neovim plugin that implements GUI helper commands
+if !has('nvim') || exists('g:GuiLoaded')
+  finish
+endif
+let g:GuiLoaded = 1
+
+" Set GUI font
+function! GuiFont(fname, ...) abort
+	call rpcnotify(1, 'Gui', 'Font', s:NvimQtToPangoFont(a:fname))
+endfunction
+
+" Some subset of parse command from neovim-qt
+" to support interoperability
+function s:NvimQtToPangoFont(fname)
+	let l:attrs = split(a:fname, ':')
+	let l:size = -1
+	for part in l:attrs
+		if len(part) >= 2 && part[0] == 'h'
+			let l:size = strpart(part, 1)
+		endif
+	endfor
+
+	if l:size > 0
+		return l:attrs[0] . ' ' . l:size
+	endif
+
+	return l:attrs[0]
+endf
+
+
+" The GuiFont command. For compatibility there is also Guifont
+function s:GuiFontCommand(fname, bang) abort
+	if a:fname ==# ''
+		if exists('g:GuiFont')
+			echo g:GuiFont
+		else
+			echo 'No GuiFont is set'
+		endif
+	else
+		call GuiFont(a:fname, a:bang ==# '!')
+	endif
+endfunction
+command! -nargs=? -bang Guifont call s:GuiFontCommand("<args>", "<bang>")
+command! -nargs=? -bang GuiFont call s:GuiFontCommand("<args>", "<bang>")
+