Rename PropertyList to PropertySet.

Avoids a name conflict with the PropertyList JS extension that would be
a problem with following commits.

Change-Id: Idef36b6eff5421fad7609462401444e552195cda
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Clean empty values from Info.plist before serializing.

Follows Xcode behavior, and prevents potential build failures due to
slight behavior change in a following commit.

Change-Id: Id0afa15c4493e460faa029fd914a19d9c3140d84
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>