Show diffs in editor

[nboldi]

This could be done when a diff arrives from server. See https://atom.io/packages/atom-unsaved-changes

Major features:

• Highlight/hide changes.
• Jump between changes.
• Finalize (apply) changes.
• If multiple diffs are received (multiple refactorings are possible) switch between them.

[nboldi]

After this is done there could be setting to always use diffs. This would make this the default behavior for every refactoring (diffs would be asked from the server).