Allow multiple overlays

This effectively allows us to support popup dialogs and the like.

Change-Id: Iafb3fa60ed635287cb59dce118f74dc8f2a4e60a
diff --git a/gui/pages.hpp b/gui/pages.hpp
index 50155d0..f0b2155 100644
--- a/gui/pages.hpp
+++ b/gui/pages.hpp
@@ -115,7 +115,7 @@
 	std::vector<Page*> mPages;
 	std::vector<xml_node<>*> templates;
 	Page* mCurrentPage;
-	Page* mOverlayPage; // This is a special case, used for "locking" the screen
+	std::vector<Page*> mOverlays; // Special case for popup dialogs and the lock screen
 	std::vector<xml_document<>*> mIncludedDocs;
 };