Include necessary files for compiling headers

This commits works towards #7289. It includes files that it depends on, or forward declares types when possible. It may seem like this only increases compilation time, but before this commit they depended on the other includes of the source files that use them.

src/openrct2/interface/Window_internal.h

@@ -17,6 +17,8 @@
 #ifndef _WINDOW2_H_
 #define _WINDOW2_H_
 
+#include "Window.h"
+
 struct rct_research_item;
 struct rct_object_entry;