[CSM] Add basic HUD manipulation. (#6067)

* [CSM] Add basic HUD manipulation.

Workaround for on_connect not working right now.

src/script/common/c_content.h

@@ -39,6 +39,7 @@ extern "C" {
 #include "itemgroup.h"
 #include "itemdef.h"
 #include "c_types.h"
+#include "hud.h"
 
 namespace Json { class Value; }
 
@@ -181,4 +182,10 @@ void               push_pointed_thing        (lua_State *L, const PointedThing &
 
 void               push_objectRef            (lua_State *L, const u16 id);
 
+void               read_hud_element          (lua_State *L, HudElement *elem);
+
+void               push_hud_element          (lua_State *L, HudElement *elem);
+
+HudElementStat     read_hud_change           (lua_State *L, HudElement *elem, void **value);
+
 extern struct EnumString es_TileAnimationType[];