WorkflowPlugin doesn't cleanup instantiated plugins

The new workflowplugin creates plugins by itself, but doesn't do any accounting of the instantiated plugins. Since they use the same plugincontext as the workflowplugin itself, cleanup also cannot be done by the framework. When editing a document, every change will trigger a new instance of the editing workflow plugin to be created. These all live side-by-side, with only the latest one visible in the UI.

The plugin framework takes care of these kind of issues, i.e. accounting of registered services and listeners, cleaning up when the plugin is shut down.

Oh, and there is an observable feature: make a change to a document, try to close the tab. Hit "save". You'll get another dialog that will generate an NPE when either save or discard is clicked.