Add support for more mouse buttons - Button8 = ESC
Please describe the problem you are trying to solve
Many mice now have extra "thumb buttons" on the side, reported by xev as button 8 and 9.
Browsers support these as history-back and forward and I'm probably not the only one who has developed a muscle memory for this.
Every time I want to exit a maximized photo, my thumb presses button 8 automatically, before I remember that Shotwell don't support this, and I have to locate and press the ESC key.
I realize that the mapping of these buttons are not self evident, as they could also be reasonably mapped to next and prev photo, but there are also buttons 6 and 7, which are "mousewheel sideways press" which would be good for this.
Shotwell 0.28.4
Ubuntu 18.04.5 LTS
Other solutions you have considered
I have hunted for ways to accomplish Button8 = ESC using gnome but gtk-can-change-accels is deprecated.
(gconf /desktop/gnome/interface/can_change_accels)
Remapping button 8 to ESC using xmodmap or similar on the system level seems risky.
I only want to touch Shotwell, and I have not found another way to do this.