Support converting a PropertyList object directly to/from JSON.

This is more efficient as there is no longer a need to convert back and
forth between strings when attempting to access the plist data.

As a side effect, this allows GUI applications to be built on
OS X < 10.7 (not that it matters).

Task-number: QBS-364
Change-Id: Ice716a2cc257efc5377049992267b927c0424d65
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>