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/CDropDown.cpp

@@ -29,7 +29,6 @@
 
 CDropDown::CDropDown(CGUI& pGUI)
 	: CList(pGUI),
-	  IGUIObject(pGUI),
 	  m_Open(),
 	  m_HideScrollBar(),
 	  m_ElementHighlight(-1),
@@ -93,7 +92,7 @@ void CDropDown::UpdateCachedSize()
 
 void CDropDown::HandleMessage(SGUIMessage& Message)
 {
-	// Important
+	// CList::HandleMessage(Message); placed after the switch!
 
 	switch (Message.type)
 	{