Sync clipboard contents on startup

[Xyene]

This patch proposes a fix for syncing correctly on startup.

Steps to reproduce (on Chrome OS):

• Ctrl+C some text in CrOS Chrome
• start Xephyr (I'm also starting i3, but can reproduce without it)
• start a terminal (for me, uxterm)
• attempt pasting in terminal
• Error: Target STRING not available is pasted into the terminal