wxMac: How to add a checkbox to a wxMessageDialog?

[macdev at gmail.com]

I wound up adding kMovableAlertWindowClass to my own build, and added
my own flag and it's working as I wanted it to now. I get a movable
modal that has no text in the title, and what would be the text title
in bold inside the dialog body itself, just like all modern OS X
dialog boxes are now.

Perhaps it's time to modify the wxDialog class to handle these new Mac
APIs, or create a new wx class altogether? I suppose I could have made
a wxModalDialog class but for now I needed something quick.