Changes

[NONE] Add TODOs to indicate label computation has to be enhanced.

[NONE] Ensure click and drag are not handled as distinct interactions.

Ensured that only one execution transaction is used for click/drag. This
had undesired effects, as there were multiple operations to undo for a
single interaction gesture.

[NONE] Ensure commit only returns an operation in case of an effect.

[NONE] Fix transactions are not properly closed by FXPinchSpreadTool.

[466815] Introduce NavigationPolicy to handle opening of graphs.

- Renamed ViewportStackModel to NavigationModel. It now keepts track of a viewport state for each graph that was left when opening a new one (nested or parent).
- Enhanced FXViewportPolicy to provide a representation of its state.
- Changed that OpenNestedGraphOnDoubleClickPolicy and
OpenParentGraphOnDoubleClickPolicy now delegate to NavigationPolicy (so navigation can be undone).