Option to float dialog windows over their parent application

[mjkillough]

Currently dialog windows like Firefox's "Are you sure you want to close these tabs?" are treated like any other. This means that if Firefox is maximised (in a stack layout), the dialog window overlays the entire Firefox window (as it becomes the focused entry on the stack), removing the context. (It's also a bit ugly).

Perhaps dialog windows should simply float in the middle of the screen, with their requested width/height? (I don't think we need to care too much about the case where their requested width/height doesn't fit on the screen).