[bug] avoid segfault when trying to show menu

for some reason I don't understand, explicit activations end up in a
segfault when I try to call menu.open() from the signal handler. i'm ok
not handling middle clicks if that means I can stop digging in search of
the real reasons, to be honest.

- Fixes: #299

Signed-off-by: kali kaneko (leap communications) <kali@leap.se>