Update xcffib version requirement

Qtile uses xcffib API for packing synthetic events that is stabilized in 0.1.8
Revert "Use .to_atoms() and .to_strings()"

After some discussion, this seems to break several things (likely a bug in
xcffib, not this commit in particular). We'll un-revert this when we fix
whatever is causing problems in xcffib.

This reverts commit bae4b9e00decd168d43201ab722f79a5be40f6ee.
ensure we .check() Checked messages

This is from 71a8d0d04602c53dd05e15a3cf8dde0a00e1fc97, but was inadvertently
removed in a758f11036c4a1f0ad83faf53933098117e6f567; we should keep it.
Use .to_atoms() and .to_strings()