true concurrent clipboard synchronization with multiple clients

[totaam]

Issue migrated from trac ticket # 1881

component: clipboard | priority: major

2018-06-20 11:13:58: antoine created the issue

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#1835 makes it possible to switch the client we synchronize with automatically, but to get clipboard contents the user must first trigger an event (click, keyboard or moving the window).
We should be able to send clipboard contents to all the clients connected, as if the server owned the clipboard when it doesn't.
The big difficulty is that some clients are greedy (win32 and macos) and some clients have more selections than others (x11) and so we would have to change the server to greedy-mode if any of the clients are greedy and use the largest set of selections then filter on the way out.