[gnome-flashback] Bug: GNOME Flashback steals Super-[a-z] key combinations

[Max Mouratov <mmouratov gmail com>]

Hello!

I've just upgraded from Ubuntu 16.04 (GNOME 3.18) to 18.04 (GNOME
3.28), and hit a major regression in handling of key shortcuts.

Any key combination of the form "Super-[a-z]" turns into "[a-z]"! For
example, "Super-p" turns into "p".

Reproducing: start the "GNOME Flashback (Metacity)" session; open 
gnome-terminal; press Super-a. Result: letter "a" gets typed.

This bug affects my usage of xmonad.