Remove virtual inheritance from the codebase, refs 6b51d71c85, e21ebb37f5 e326ebae46.

Remove workaround in 3d07327837 / D2136, the pointer from IGUIObject to
IGUITextOwner that avoided the dynamic_cast.
Have the GUIObject implementing class take responsibility of calling the
virtual methods in the base classes.
Leaves the code in a clean state that can serve as an example for new
GUI object classes, such as D1346, refs #4683.

Differential Revision: https://code.wildfiregames.com/D2325
Comments By: Vladislav on IRC 2019-07-30, 2019-08-22, 2019-09-20-23
Tested on: clang 8.0.1, Jenkins

This was SVN commit r23020.

source/gui/IGUIObject.cpp

@@ -453,6 +453,8 @@ void IGUIObject::CreateJSObject()
 
 	m_JSObject.init(cx, m_pGUI.GetScriptInterface()->CreateCustomObject("GUIObject"));
 	JS_SetPrivate(m_JSObject.get(), this);
+
+	RegisterScriptFunctions();
 }
 
 JSObject* IGUIObject::GetJSObject()
@@ -465,6 +467,11 @@ JSObject* IGUIObject::GetJSObject()
 	return m_JSObject.get();
 }
 
+bool IGUIObject::IsEnabled() const
+{
+	return m_Enabled;
+}
+
 bool IGUIObject::IsHidden() const
 {
 	return m_Hidden;