Implement dynamic list subscription and a list that tracks its elements

[oowekyala]

So there's something that I needed and I think it can be useful in the main repository. Changelog:

• Add LiveList#flattenVals: basically this can be viewed as a transformation ObservableList<Val<T>> -> LiveList<T>, it "flattens" the Val context into the list. I guess the transformation would be functionally equivalent to _.flatMap(LiveList::wrapVal)
  • The returned list observes the changes of its individual elements and outputs them as ListChanges.
  • Example use case:

LiveList<BlogEntry> entries = ...;

// the size property is now also updated when any BlogEntry.wasReadProperty() changes value
Val<Integer> readBlogs = entries.flattenVals(BlogEntry::wasReadProperty)
                                .filter(Boolean::booleanValue)
                                .sizeProperty();

• The implementation needed implementing a dynamic subscription function for ObservableLists, which is in fact quite useful! I think that has the potential to e.g. simplify implementation of Implement ObservableList.flatMap #53