Fix a9e845 and 797755 Allow X*IfEvent() to reenter libX11

a9e84580 tried to fix 79775575 but introduced a condition where another thread could increase in_ifevent before waiting for the other thread to unlock the display and also prevented other threads from locking / unlocking the display. This commit tries to implement 79775575 with a simpler approach which hopefully fixes all issues mentioned in #168 (closed).
Note that I only tested this with firefox on my personal machine and it should be made sure that this still fixes the original problems described in !150 (merged).