Make QPlaceContent extendable by third parties

QPlaceContent is currently limited to the types of reviews, editorials and images. The API should be modified to allow the possibility of 3rd parties to introduce their own custom content types.

This would be achievable by adding a key-value map field to QPlaceContent where third parties can add their own custom data. A CustomType enum could be added to QPlaceContent::Type, in conjunction with an int QPlaceContent::customType() function, this would operate in a similar manner to the QVariant::UserType enum and int QVariant::userType() function.