Perfected selection/deselection. Added support for boxes alignment when dragging. Bug fixes.

Experiment editor summary up to this commit:
* Everything related to selection/deselection now works, with the
exception of the old multi-selection bug:
- selecting/deselecting a single box,
- selecting/deselecting additional boxes with the SHIFT key down.
* When dragging items, alignment lines are displayed instead of edges.
* Edges are correctly updated on drag operations.
* Boxes can be removed with the BACKSPACE key.

TODO:
- undo/redo operations
- serialization/deserialization from/to JSON