Fix most headers in gui

Make include-what-you-use happy with most of the files in source/gui and
fix what needs to be fixed after including missing compile flags.

Ref: #8086
Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

source/gui/Scripting/JSInterface_GUIProxy.cpp

@@ -17,6 +17,7 @@
 
 #include "precompiled.h"
 
+#include "JSInterface_GUIProxy.h"
 #include "JSInterface_GUIProxy_impl.h"
 
 #include "gui/ObjectBases/IGUIObject.h"
@@ -26,6 +27,11 @@
 #include "gui/ObjectTypes/CScrollPanel.h"
 #include "gui/ObjectTypes/CText.h"
 
+#include <string>
+
+class CGUIString;
+class ScriptRequest;
+
 // Called for every specialization - adds the common interface.
 template<>
 void JSI_GUIProxy<IGUIObject>::CreateFunctions(const ScriptRequest& rq, GUIProxyProps* cache)