Add getTextSize() for CButton

Summary: 9c5062147a introduced the `getTextSize()` to all GUIObjects for
getting the rendered size of a text. 8190293f8b made it only available
to CText. `getTextSize()` can also be useful for CButtons so this diff
adds the function back to CButtons. There seem to be no other GUIObjects
besiders CText and CButton where `getTextSize()` makes sense.

Reviewed By: wraitii
Differential Revision: https://code.wildfiregames.com/D3195
This was SVN commit r24378.

source/gui/GUIObjectTypes.cpp

@@ -40,6 +40,7 @@ void CGUI::AddObjectTypes()
 	m_ProxyData.insert(JSI_GUIProxy<IGUIObject>::CreateData(*m_ScriptInterface));
 	m_ProxyData.insert(JSI_GUIProxy<CText>::CreateData(*m_ScriptInterface));
 	m_ProxyData.insert(JSI_GUIProxy<CList>::CreateData(*m_ScriptInterface));
+	m_ProxyData.insert(JSI_GUIProxy<CButton>::CreateData(*m_ScriptInterface));
 
 	AddObjectType("button", &CButton::ConstructObject);
 	AddObjectType("chart", &CChart::ConstructObject);