FXCollections should provide a factory method for creating observable hash maps

FXCollections currently defines a factory method for creating observable array lists (FXCollections#observableArrayList()) but does not define a similar method for hash maps. As a result, it is not currently possible to create an observable hash map in FXML, which uses the following syntax for invoking no-arg factory methods:

<FXCollections fx:factory="observableArrayList"/>

Ideally, we should also be able to write:

<FXCollections fx:factory="observableHashMap"/>

to create an observable HashMap.