Rework internal Preview and XRC test functionality

[KeyWorksRW]

Description:

Internal builds have the ability to preview dialogs either using C++ or XRC. In addition, there is an XRC test dialog that also contains an XRC preview. It would make sense to move the XRC/C++ comparison into the XRC test dialog, and have the Preview button always display the C++ version (without asking first). That would make it reasonable to start working on getting that reliable enough to use in a release build.

Note that making the XRC test dialog pickup the functionality, it should have a button that automatically generates and previews based on the currently selected node/form in the Navigation panel. Making this a Window like our Log window would allow us to walk through the forms with the preview being automatically updated.