Add snap to grid and keyboard navigation support for selected nodes/groups

[cneben]

• [x] Add "snap to grid" support in qan::DraggableCtrl.

  • [x] Add a snapToGrid property to qan::Graph.
  • [x] Add a gridSize property to qan::Graph.

• [x] Add support for selection keyboard navigation (moving).

  • [ ] Ensure it work with node/groups heterogeneous selections.
  • [ ] Ensure snapToGrid is taken into account.

• [x] FIX: qan::EdgeDraggableCtrl is no longer working with snapping and snap orientation constrains...

• [ ] Fix all FIXME #185