preparing the previewsmanager class

--how to pass signals from the workflow.cpp to qml plugin
must be investigated probably with connections through
the qml (PreviewsConnections class could be created)
enable the previewManager instead of taskManager for previews

--this is one step before moving taskManager and previewManager
to qml plugins
added the needed connections for previewManager
fixed bug of activating a window
clean up and update qml for taskManager