translate vietnamese strings:

    # _layout/classic_joystick_preferences.rpym:94
    old "Joystick Mapping"
    new "Hệ thống nút tay cầm"

    # _layout/classic_load_save.rpym:138
    old "Empty Slot."
    new "Ô còn trống"

    # _layout/classic_load_save.rpym:170
    old "a"
    new "a"

    # _layout/classic_load_save.rpym:179
    old "q"
    new "q"

    # _compat/gamemenu.rpym:355
    old "Previous"
    new "Về trước"

    # _compat/gamemenu.rpym:362
    old "Next"
    new "Về sau"