Add ability to integrate another form as a child in the current form

[KeyWorksRW]

Description:

The upside of creating a form version of wxPanel, wxToolBar, wxMenu, etc. is that it avoids having an extremely large source code file. The downside is that you have to integrate it yourself -- there's no easy way to integrate one form into another form using wxUiEditor.

It should be possible to provide integration. We would already know how to generate the code to create a new instance. We could even have the Property Grid point to the form node's properties. We could figure out what header file or module needs to be included in the source file as well as class forward definition in the C++ header file.