Restore state support for command handlers as documented in
IObjectWithState Javadoc.

See
https://bugs.eclipse.org/bugs/show_bug.cgi?id=549802

Fixes #925

Command state support was lost during migration on E4 as proxies failed
to properly support IObjectWithState protocol or ignored is altogether.

- HandlerProxy
- HandlerServiceHandler
- E4HandlerProxy

do now support the protocol. Following tests are now reenabled:

- StateTest
- ToggleStateTest

Additional integration test added:

- WorkbenchStateTest