Raise Qt dependency to >= 4.8; raise kdelibs dependency to >= 4.8.4

...as agreed upon on amarok-devel ML [1], see the thread for reasons.

[1] http://mail.kde.org/pipermail/amarok-devel/2012-November/011218.html

DIGEST: Amarok now depends on Qt >= 4.8, kdelibs >= 4.8.4
CCMAIL: amarok-devel@kde.org