ExportDlg: Minor refactoring: input must be passed to constructor instead of passed in later.
This is to support new lazyflow assertion regarding operator relationships for connected slots.
datasources: Added abtract method SourceABC.clean_up()
Layer: Added clean_up() method
LayerStackModel: Call Layer.clean_up() when a layer is removed from the model.
(This means that LayerStackModel the explicit owner of all Layers it contains.)
LazyflowSource: Init op5 member with both parent and graph, in case parent is None.
layerwidget: When simulating a second click during mousePressEvent, use QApplication.sendEvent instead of calling the protected mousePressEvent() and mouseReleaseEvent() methods.