hookup the mouse back button to the 'app.back' action
This change allows the mouse back button to trigger 'back' where the action already exists. Implements enhancement in #415 (closed).
I'm fairly new to gtk-rs, GTK+ and GUI development in general, just FYI.
I don't really know if there was a better way to go about doing this, I had initially hoped to just add something to https://gitlab.gnome.org/GNOME/fractal/blob/aafd8096c596d4ec9ddd2389b59ff7b3f841f00d/fractal-gtk/src/actions/global.rs#L228, but that seems to only work with keyboard key-presses, not mouse buttons.
Also, I'm not sure if the "8" (button number for back on mouse) has a const/enum value anywhere, but I couldn't find one. Although since EventButton::get_button just returns a u32, I'm guessing there isn't one.