Qt: Fix button in speed of subtitles.
Qt: Fix button in speed of subtitles.

"Adjustment and Effects, Synchronisation, Subtitles/Video, Speed of the subtitles, minus sign is not clickable" Spotted by Trax.

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