Fix `xsel` import when clipboard is empty, and add timeout failsafe

[clayote]

My Kivy installation was hanging on import, and it turned out to be because xsel wasn't sending anything through its pipe -- because the selection was empty. It looked like xsel only got called to make sure it was there, so I added a --version argument, which will return something with 0 exit code if xsel is available.

I also added a timeout to the subprocess, because it seems bad that kivy just hangs when the subprocess does. If xsel is broken, Kivy should just use another clipboard provider, rather than locking up.

I removed the bare except because it did nothing.