Use unified LoadFileToBuffer function

Create a single function for loading xml files to a memory buffer
to reduce code duplication.

Change-Id: I0ff64463072118f61a2eb8052f74019715020253
diff --git a/gui/pages.hpp b/gui/pages.hpp
index 81112f9..03fb0e5 100644
--- a/gui/pages.hpp
+++ b/gui/pages.hpp
@@ -123,6 +123,7 @@
 {
 public:
 	// Used by GUI
+	static char* LoadFileToBuffer(std::string filename, ZipArchive* package);
 	static int LoadPackage(std::string name, std::string package, std::string startpage);
 	static PageSet* SelectPackage(std::string name);
 	static int ReloadPackage(std::string name, std::string package);