Add support for paravirtualized Clipboard and diffferent screen sizes

Backported from my Oberon emulators written in Java and JavaScript, available from
http://schierlm.github.io/OberonEmulator/

Note that you will need to patch Display.Mod and need a Clipboard.Mod to make use of these changes. See the commit comments for the locations of these files.

The patched Oberon image should still work on real hardware (not tested since I don't have it, though).