[qt4-build-multilib.eclass] New multilib-compatible eclass for building Qt4.

For now it's just a copy of qt4-build.eclass with tons of cleanups, removal
of dead code and old cruft, and general function reorganization to improve
readability. Real multilib support will be added soon. See also bug 498010.

One major change of this eclass is that all Qt4 binaries are now installed
into '/usr/$(get_libdir)/qt4/bin', similarly to Qt5.