Add Debian dependency: touchegg

[m-roberts]

At least a recommends field for it would make sense, even if not a hard dependency. This would mean that if touchegg is not installed and touche is installed from a Debian repository, then touchegg will be installed also. I think that this is the ideal behaviour.

[JoseExposito]

Yes, this could be really convenient. However, I need to do some testing, because the package available in the Ubuntu repos (https://packages.ubuntu.com/groovy/touchegg) is really outdated and I don't want to pull that one by mistake.
Ideally that package should be updated, but it doesn't seem like there is any interest on it.