possibly fixes issue #64, opens popup on keyup

This perhaps fixes issue #64.

I've looked how xfce4-applicationsmenu does it. It's not perfect. I won't say I understand the code, but the behavior is somewhat quirky: If `<Super>` is bound to whiskermenu popup and `<Super>+T` to Terminal, `<Super>+T` opens up Terminal as expected, but also whiskermenu if `<Super>` is released within 1/4 of a second (as defined in the auxiliary function).

How to improve it, I haven't got a clue. :smiley_cat: