qt: replaced magic constants with QtAbstractSlider::{maximum, minimum}
qt: replaced magic constants with QtAbstractSlider::{maximum, minimum}

Replaced the use of constants with calls to QAbstractSlider::maximum and
QAbstractSlider::minimum, making it easier to maintain the class in the
future.

Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc,xkfz007/vlc