Ability to create child windows

[timboudreau]

It would be very helpful if there were some way from baseview to create actual child/secondary windows - i.e. to implement controls such as this.

There would likely need to be some additional abstraction the caller would have to use to poll for messages to the window, or something like that, but it seems relatively doable.

If your UI design philosophy is not simulating analog hardware with knobs, they are rather essential.

[timboudreau]

FYI, this was trivial to implement for Mac OS. It also adds optional settings for specifying whether the window should be borderless, and a few other Mac OS specific window options, so popups can be created without a title bar.

I could likely do similar for xwindows on Linux, but have no machine to test a similar patch for Windows - if anyone would be willing to contribute one, this could be made into a pull request.