The `set' event already existed---this merely extracts it into its own
handler.
* src/client/Client.js (handleEvent): Extract `set' handler.
* src/client/ClientDependencyFactory.js (createClientEventHandler): Add
`set'.
* src/client/event/ValueSetEventHandler.js: New class.
* test/event/ValueSetEventHandlerTest.js: Associated test case.