Dslide Present Command, Multi-Frame Support

[psionic-k]

It would be smart to detect if there are multiple screens and to generally open the presentation on the "other" screen, offering to swap them and make one or both fullscreen etc.

This requires decent DE integration to make the decision. Sadly my combination of software seems to result in Emacs using the fallback method to detect screens even though I have xrandr installed. I'm blocked until I can decide how to place the frames correctly.

More thoughts found on #2