tell the server about clipboard direction

[Antoine Martin]

Split from #276 (see ticket:276#comment:15): OSX clients receive the token and the data at the same time, we should tell the server about client-side direction settings so that the server can avoid sending that data.
This could be considered a security issue: when clients request "to-server" only, they don't expect server clipboard data to travel in the other direction at all...

The "can_send" and "can_receive" flags would have to be split between client and server side settings, with the former being changed at runtime through a new clipboard packet type (new capability). Tricky.