Add register dialog to separate login/register (#12185)

New users find Minetest's account system confusing.
This change moves username/password to a new dialog,
with login and register buttons added to the Join Game tab.

The old registration confirmation dialog is removed in
favour of the new dialog.

Fixes #8138

src/gui/guiMainMenu.h

@@ -20,6 +20,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #pragma once
 
 #include "irrlichttypes_extrabloated.h"
+#include "gameparams.h"
 #include <string>
 #include <list>
 
@@ -50,5 +51,7 @@ struct MainMenuData {
 	// Data to be passed to the script
 	MainMenuDataForScript script_data;
 
+	ELoginRegister allow_login_or_register = ELoginRegister::Any;
+
 	MainMenuData() = default;
 };