Subclasses of object workflow are responsible for different inputs.

Removes lots of code duplication.
fixed import in blockwise classification
serializers ignore dirty signals during deserialization

fixes #362
a little cleanup of AppletSerializer
workflows may be initialized with kwargs
synapse workflows replaced with kwargs
update "recently opened files" when switching to startscreen
carving: do not access raw data via MST
dataSelection: handle right click on table view: fix #366
Fix indentation
LabelingApplet: Avoid instantiating an unused top-level operator if the subclass has already created a top-level operator.
CarvingGui: Don't assume that RawInput is always ready().
Carving:
- Set correct dtype for output slots.
- Expose MST as a top-level slot to allow svg visualization
- Hard-code 'labels allowed' flag as True, since it makes no sense to open the carving workflow without labeling it.
Added CarvingFromPixelPredictionsWorkflow
OpPreprocessing: Refactored as a composite of three internal operators: filter, watershed, and mst creator.
PreprocessingGui: Show filtered output, watershed and layer controls.
add "block gui" to layer viewer menu in debug mode