Implement relative positioning system for DialogXML

This allows widgets to be positioned relative to other widgets.

Two dialogs have been converted to use relative positioning.

Also fix some issues with calculating the dialog rect.

src/dialogxml/widgets/stack.hpp

@@ -68,7 +68,7 @@ public:
 	/// @return The number of pages
 	size_t getPageCount();
 	/// Recalculate the stack's bounding rect based on its contained controls.
-	void recalcRect();
+	void recalcRect() override;
 	/// Adds any fields in this stack to the tab order building arrays.
 	/// Meant for internal use.
 	void fillTabOrder(std::vector<int>& specificTabs, std::vector<int>& reverseTabs);