hard-dep for Qt 4.8

[Shaun Reich]

Prompting motion for making Qt 4.8 a hard dependency for KDE
4.9/master. Currently kde-baseapps/plasma/folderview does not build
against 4.7 due to an (albeit minor) api usage.

Not sure how other areas in KDE fair against this, most notably the
Plasma area (with QML and what not).

If we depend on it, we need to set the CMake check obviously.

-- 
Shaun Reich,
KDE Software Developer (kde.org)