window: Go back/forth views using dedicated hardware buttons
Refactor on_window_previous_date_activated_cb and on_window_next_date_activated_cb into separate methods for going to the prev/next view, to be used in the callbacks for both the forward/back buttons in the UI, and for the Gtk.GestureClick callbacks that listens for the hardware buttons to be pressed.
Closes: #1494 (closed). Thank you @jfft for the tip in the original issue :)