[v3 linux] systray implementation

Linux requires a `gtk_menu_bar` for a gtk_window to display a menu.
For the `systray` a `gtk_menu` is needed instead.
This change creates the correct type of `impl` for the `Menu`
depending on how it is being used.

v3/pkg/application/application_linux.go

@@ -82,6 +82,7 @@ func (m *linuxApp) getApplicationMenu() pointer {
 
 	menu := globalApplication.ApplicationMenu
 	if menu != nil {
+		menu.impl = newMenuBarImpl(menu)
 		InvokeSync(func() {
 			menu.Update()
 		})