Fix issue with clipboard manager

Hi Zbigniew,

I came up with the following fix for the clipboard-manager issue reported by multiple users:
https://github.com/zdia/gorilla/issues/108
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=629114

I'm not sure if this the best way to solve the issue, but I can't think of anything better...

Alexandre

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Don't consider clipboard 'paste' events occurring less than 200ms
after a clipboard 'copy' event as having been triggered by a user.