Improve Info.plist handling.

It's now possible to read from existing Info.plist files. The data in
an external plist file will be aggregated with and overridden by the
object dictionary specified in the QBS file.

Postprocessing can also be disabled if desired.

Defaults handling has also been DRY'ed up significantly and made more
consistent across different bundle types and platforms.

Task-number: QBS-301
Change-Id: Id3ff3bbca1e02c467143513216ef66d38ec9aa6b
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>