This should "protect" the user from some stupidities that Lua code might be
doing that e.g. makes a client jump to another position and then immediately
back to where it was before. Only the last change in a single main loop
iteration will actually have any effect.
Original idea by Daniel here: https://github.com/awesomeWM/awesome/pull/174
Signed-off-by: Uli Schlachter <psychon@znc.in>