osx: fix signing to make Gatekeeper happy (again)

The approach from 65f3fa8d1 worked for signing on 10.9.4, but not newer
versions. 10.9.5 (and up) want each framework to stand alone.

Now in addition to copying the plist's from Qt for each framework, we put them
in per-version dirs and only symlink to the latest, rather than using symlinks
for any contents.