Add support for creating an MDI application

[KeyWorksRW]

Description:

Creating an MDI application is quite a bit more complicated than the more common SDI approach. While the frame window won't really benefit much from a RAD tool, documents and views, can definitely benefit -- particularly for their menus.

I've already got some code in place for this, but not surprisingly it's going to take a lot of coding to make this generally useful as well as customizable. This issue is being added so that we can track the issue.

Note that we are unlikely to be able to complete this in time for the 1.1.1 release in the second quarter of this year. We can leave the existing code in place, but before release, we'll need to shut off the UI in the Ribbon panel.