make monodevelop depend on xterm or debugging will not work.

See
https://bugs.launchpad.net/ubuntu/+source/monodevelop/+bug/1090977
and
https://bugs.archlinux.org/task/28898

This is still valid for monodevelop-4.0.
So I changed the ebuild and added a (runtime) dependency for xterm.
I added -r1 because this way portage will notice the dependency when 4.0 is already installed.