Make code compatible with gnome-shell 3.32
Created by: ernestask
This commit ports class declarations to those of ES6[0], drops the convenience module[1] and updates function context binding[2], adopting the arrow notation in places.
[0] - https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/361 [1] - https://gitlab.gnome.org/GNOME/gnome-shell-extensions/commit/bab4be1a597226a5fe8a29dcf3848be47e16a5e5 [2] - https://gitlab.gnome.org/GNOME/gnome-shell-extensions/commit/ba27cc4a64daf7f62174653362741de30c2f5b86
Fixes https://github.com/projecthamster/hamster-shell-extension/issues/307