Warn if our event processing is falling behind

Take a "random" time snapshot at libinput_dispatch() and compare that to the most recent evdev timestamp. Where the timestamp is more than 100ms in the past print out a warning that we're not processing events fast enough. This complements the timer offset warning which does the same thing but obviously only for timers. This warning should show up for any event.

It needs to be disabled for the test suite though because with the hand-crafted event/dispatch sequences there we'd trigger it in many cases and fixing that is too much effort for now.

cc @csoriano, for no special reason other than that he recently ran into a problem like that :)