event_curtime is replaced (publicly) by event_time()

This function never returns CurrentTime, which is nice, cuz using CurrentTime for XSetFocus always sucks.

If the current XEvent did not have a timestamp, then event_time() will find one.  It finds the first timestamp available in the X event queue, meaning the earliest timestamp >= the current (nontimestamped) event. All future events should have a timestamp >= event_time(), so using this in XSetFocus() should not mess up any future calls we make to it.

This change seems to work well, as it appears to fix bug #3648.