We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
A SPICE agent communicates with the host OS to provide nifty features
like clipboard sharing :^)
This patch implements only plain-text clipboard sharing.
See: github.com/freedesktop/spice-protocol/blob/master/spice/vd_agent.h
