Qt4: limit the UI volume to 125% (about +6dB)

Far too many clueless users push the volume far too high[1]. Then they
complain that VLC distorts the sound... Really anything above 100% (0dB)
is prone to saturation. In my experience, loss of quality is noticeable
at +6dB and unbearable at +10dB.

For the references:
In VLC 1.1, the UI was restricted to 200% (+6dB) by default,
while the audio output core allowed up to 400% (+12dB).
PulseAudio recommends +11dB.
Microsoft Windows limits to +0dB.

[1] I actually compared VLC 1.1, VLC 2.0 and WMP with an external
decibel metter. They give the same output at 0dB. At 6dB, both VLC
versions give the same decibel readings and slight distortion. So the
complaints are purely caused by confusion and cluelessness from users.

modules/gui/qt4/util/input_slider.cpp

@@ -433,7 +433,7 @@ bool SeekSlider::isAnimationRunning() const
 #define WLENGTH   80 // px
 #define WHEIGHT   22  // px
 #define SOUNDMIN  0   // %
-#define SOUNDMAX  200 // % OR 400 ?
+#define SOUNDMAX  125 // % (+6dB)
 
 SoundSlider::SoundSlider( QWidget *_parent, int _i_step,
                           char *psz_colors )