Qt: SoundSlider: fix text color contrast. (closes #5040)
Qt: SoundSlider: fix text color contrast. (closes #5040)

Uses half hsv's value between background and foreground
(instead of mid palette color which is only revealant for drawing over
buttons colors)

vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc,krichter722/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,vlc-mirror/vlc-2.1,xkfz007/vlc,krichter722/vlc,xkfz007/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc,jomanmuk/vlc-2.2,krichter722/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.2,shyamalschandra/vlc,vlc-mirror/vlc,krichter722/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc,xkfz007/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,vlc-mirror/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,vlc-mirror/vlc-2.1