add an indicator icon for the current activity's icon

--when the current activity's icon is set in the panel
SUCCESS: engines moved to qml plugins

--the change was easy so the following engines
are now qml plugins: taskManager, previewsManager,
workflowManager(contains activityManager, workareasManager
and the ActivitiesEnhanchedModel)
--The ActivitiesEnhanceModel contains all the added
functionality to activities that is workareas, backgrounds,
and in the future ordering, enabling/disabling workarea