add support for wayland

Currently break-time only supports X, not Wayland. It would be nice to extend break-time to also be usable on Wayland.

The X-specific places in the code can be found by grepping for `xcb` (which is the Rust library binding to the XCB system library, used for manipulating and querying X).

I'm not sure the best way to support both X and Wayland, but maybe all of the X-specific code could be pulled to a single module, and have a separate module with the same interface for Wayland. These could be switched at compile-time with Rust feature flags.

A similar thing could be done to support other platforms like OSX or Windows, but that is somewhat out of scope for this issue.