The x11 backend exposes a lot of symbols that are meant to only be used
when implementing a subclassed backend, like the glx and eglx ones.
The uninstalled headers are also filled with cruft declarations of
functions long since removed.
Let's try to clean up this mess.
If we have XKB support then we should be using it to turn on the
detectable auto-repeat; this allows avoiding the peeking trick
that emulates it inside the event handling code.
