Capture and forward mouse buttons 8 and 9

These buttons are not mapped by GTK
(`/usr/include/gtk-3.0/gdk/gdktypes.h`) but still exposed, so I
hardcoded their values:

    #define _GTK_BUTTON_PREV_MASK (1 << 15) /* button 9 */
    #define _GTK_BUTTON_NEXT_MASK (1 << 16) /* button 8 */