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/gameparams.h

@@ -20,8 +20,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #pragma once
 
 #include "irrlichttypes.h"
-
-struct SubgameSpec;
+#include "content/subgames.h"
 
 // Information provided from "main"
 struct GameParams
@@ -34,6 +33,12 @@ struct GameParams
 	bool is_dedicated_server;
 };
 
+enum class ELoginRegister {
+	Any = 0,
+	Login,
+	Register
+};
+
 // Information processed by main menu
 struct GameStartData : GameParams
 {
@@ -46,6 +51,8 @@ struct GameStartData : GameParams
 	std::string address;
 	bool local_server;
 
+	ELoginRegister allow_login_or_register = ELoginRegister::Any;
+
 	// "world_path" must be kept in sync!
 	WorldSpec world_spec;
 };