Bug 218884 [DataBinding] Add API: WorkbenchObservables
... for things like listening to ISelectionService
bundles/org.eclipse.core.databinding.observable/src/org/eclipse/core/databinding/observable/Observables.java
* observable map's entry specified by its key.
	 * <p>
	 * The state where the key does not exist in the map is equivalent to the
	 * state where the key exists and its value is <code>null</code>. The
	 * transition between these two states is not considered a value change and
	 * no event is fired.
	 * 
	 * @param map
	 *            the observable map whose entry will be tracked.
	 * @param key
	 *            the key identifying the map entry to track.
	 * @return an observable value that tracks the value associated with the
	 *         specified key in the given map
	 * @since 1.2
	 */
	public static IObservableValue observeMapEntry(IObservableMap map, Object key) {
		return observeMapEntry(map, key, map.getValueType());
	}
	/**
	 * Returns an observable value that tracks changes to the value of an
	 * observable map's entry specified by its key.
	 * <p>
	 * The state where the key does not exist in the map is equivalent to the
	 * state where the key exists and its value is <code>null</code>. The
	 * transition between these two states is not considered a value change and
	 * no event is fired.
	 * 
	 * @param map
	 *            the observable map whose entry will be tracked.
	 * @param key