implement new clipboard protocol
This includes VD_AGENT_CAP_CLIPBOARD_NO_RELEASE_ON_REGRAB and VD_AGENT_CAP_CLIPBOARD_GRAB_SERIAL.
Without VD_AGENT_CAP_CLIPBOARD_NO_RELEASE_ON_REGRAB, spice-gtk has to wait for 0.5 seconds to see whether the release message was because of a regrab or not, which might not always be reliable (but it does not seem to cause any issues atm).