Add a property allowing the user to specify the Info.plist format for their bundle

Xcode allows the Info.plist output format to be chosen by the user; QBS should provide this feature as well.

Currently, we write xml1 format for OS X, and binary1 for iOS (hardcoded in DarwinGCC.qbs).

We should have a (string) cpp.plistFormat property whose valid values are: [ "xml1", "binary1", "json", "same-as-input" ]

If the user leaves the value undefined we should follow Xcode's behavior:

• OS X: the default value should be "same-as-input" (if cpp.infoPlistFile is defined) or "xml1" (if it is not)
• iOS: the default value should be "binary1"