Allow to cancel item modifications

[jrmi]

In the left pane, there is only the "X" icon to close it. And you can also close by clicking elsewhere than in the left pane. In both cases, our changes are saved.

Problem: how to undo bad handling?

Solution: add a validation icon. And if we click on the "X" icon or if we click outside the left pane, a message appears asking to confirm our choice not to save our changes.