[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/menu_linux.go

@@ -8,6 +8,14 @@ type linuxMenu struct {
 }
 
 func newMenuImpl(menu *Menu) *linuxMenu {
+	result := &linuxMenu{
+		menu:   menu,
+		native: menuNew(),
+	}
+	return result
+}
+
+func newMenuBarImpl(menu *Menu) *linuxMenu {
 	result := &linuxMenu{
 		menu:   menu,
 		native: menuBarNew(),