Add neovim#gui api (#2368)

* Add neovim#gui api

* Add gui function

autoload/SpaceVim/api/neovim/gui.vim

@@ -0,0 +1,34 @@
+let s:self = {}
+
+
+function! s:self.gui_name() abort
+  if !has('nvim-0.3')
+    return ''
+  endif
+
+  let uis = nvim_list_uis()
+  if len(uis) == 0
+    echoerr "No UIs are attached"
+    return
+  endif
+
+  " Use the last UI in the list
+  let ui_chan = uis[-1].chan
+  let info = nvim_get_chan_info(ui_chan)
+  return get(info.client, 'name', '')
+endfunction
+
+
+function! s:self.toggle_tabline(enable) abort
+  call rpcnotify(0, 'Gui', 'Option', 'Tabline', a:enable)
+endfunction
+
+function! s:self.toggle_popupmenu(enable) abort
+  call rpcnotify(0, 'Gui', 'Option', 'Popupmenu', a:enable)
+endfunction
+
+function! SpaceVim#api#neovim#gui#get()
+
+  return deepcopy(s:self)
+
+endfunction