New dependency on Qt 4.8

Due to the removal of "#ifdef USE_GLYPHS" (as of revision 4962), MuseScore now depends on Qt 4.8. Cause is the qpainter->drawGlyphRun() call in trunk/mscore/libmscore/sym.cpp:505, which is now unconditionally used.

qpainter->drawGlyphRun() was introduced in Qt 4.8.

As Qt 4.8 is not yet released, this seems not desired. Furthermore, CMakeList.txt still contains set(QT_MIN_VERSION "4.7.0")

I would prefer to delay depending on Qt 4.8 at least until it is released.