Adds 'exposed' field to ClassInfo

This field represents if the class is exposed to the scripting API.
The value is 'true' if the class was registered manually ('ClassDB::register_*class()'), otherwise it's false (registered on '_post_initialize').
- Added missing registration of classes that are meant to be exposed.

core/class_db.h

@@ -127,6 +127,7 @@ public:
 		StringName inherits;
 		StringName name;
 		bool disabled;
+		bool exposed;
 		Object *(*creation_func)();
 		ClassInfo();
 		~ClassInfo();
@@ -168,6 +169,7 @@ public:
 		ClassInfo *t = classes.getptr(T::get_class_static());
 		ERR_FAIL_COND(!t);
 		t->creation_func = &creator<T>;
+		t->exposed = true;
 		T::register_custom_data_to_otdb();
 	}
 
@@ -176,6 +178,9 @@ public:
 
 		GLOBAL_LOCK_FUNCTION;
 		T::initialize_class();
+		ClassInfo *t = classes.getptr(T::get_class_static());
+		ERR_FAIL_COND(!t);
+		t->exposed = true;
 		//nothing
 	}
 
@@ -193,6 +198,7 @@ public:
 		ClassInfo *t = classes.getptr(T::get_class_static());
 		ERR_FAIL_COND(!t);
 		t->creation_func = &_create_ptr_func<T>;
+		t->exposed = true;
 		T::register_custom_data_to_otdb();
 	}
 
@@ -347,6 +353,8 @@ public:
 	static void set_class_enabled(StringName p_class, bool p_enable);
 	static bool is_class_enabled(StringName p_class);
 
+	static bool is_class_exposed(StringName p_class);
+
 	static void add_resource_base_extension(const StringName &p_extension, const StringName &p_class);
 	static void get_resource_base_extensions(List<String> *p_extensions);
 	static void get_extensions_for_type(const StringName &p_class, List<String> *p_extensions);