Moving frames around screens

[Psionic K <psionik@positron.solutions>]

I'm adding a command to dslide called `dslide-present'.  It will
create a frame that has a modal transient UI for positioning the
frame.  Since the user won't control both frames simultaneously. the
base frame and presentation frame should be able to swap positions

This is the kind of problem that really relies on experience with
different desktop environments.  Are there certain functions and
strategies I should rely on or avoid to make this work well on
multiple desktop environments?

Thanks