Avoid leak of updates in property caching code

The following discussion from !383 (merged) should be addressed:

• @danieldg started a discussion: (+1 comment)

  A full fix for this issue may require a separate channel for each property. If you do that, you can replace the broadcaster with an (OwnedValue, EventListener) pair per property, as part of the property cache.

  For uncached/uncacheable properties, you might need to change the cache to look something like:

  struct {
     value: Option<OwnedValue>,
     cacheable: bool, // or enum { NotCacheable, Cacheable, Constant }
     changed: Event, // sent after setting the Option; the channel listens to it and then reads.
     // An invalidation PropertyChange signal can cause it to read a value of None.
  }

  You could also do something a bit fancier that lets you discard the value if it's not supposed to be cached and all listeners have picked it up (or if there are no listeners).