Added the option to have user defined filterable properties. These are base types (short, int, long, long long, float, double, string) which can be specified by the user in addition to the properties defined by the object themselves. These are used in the message headers of the Events that are sent. These can be used in conjunction with selectors to filter server-side, rather than having the application do it.
These changes are now in place, and I'm going to move this to "In Review" when the rest of the related tickets are completed, so it can be reviewed all at once because of the number of changes that have been made. This work is under ctrl_events/tickets/dm-1350.