Add Preview sub-panel to XRC panel

[KeyWorksRW]

Description:

In the XRC panel, it would be useful to create a third panel called "Preview" that would display a live preview of the XRC code. Forms that could be displayed in a wxPanel would be displayed live. If the form cannot be displayed in a wxPanel, then text indicating that it can't be displayed within the panel, but providing a button to open the form in a top level window could be shown.