node_properties_changed should be used where appropriate

[irgolic]

(transferring from #191 so it doesn't get lost)

Until now, canvas-core was not aware of changes to settings in widget-base. It only knew to call sync_node_properties to pack Settings when saving. We have added a node_properties_changed signal, intended to be emitted by subclasses of Scheme. (for widget-base, this is implemented in biolab/orange-widget-base#151)

The following improvements should be relatively trivial to implement. There's probably some more uses for it elsewhere.

• save crash recovery state also on Setting change, not just workflow change (CanvasMainWindow.save_swp). Also, the workflow should freeze when restoring crashed state, so a perpetually crashing state is not restored.
• set SchemeEditWidget as modified on Setting change. With this change, Orange should be acutely aware of when the workflow is and is not modified. No more 'do you want to save?' when no changes have been made.