Remove plugins and themes that were not present on restored backup

At the moment, if you make a backup and later install new plugins and new themes, when you come to restore that backup (which did not have those plugins and themes installed at the time), the new plugins and themes are still there.

This is not how a backup and restore should work. Those new plugins and themes should be removed.

If I have backed up my site at a time when I did not have certain themes and plugins installed, and I restore that backup, that might be because I do not want those themes or plugins anymore because they are causing issues.

With the current way backup and restore works, plugins and themes I do not want anymore are still on my site, even though I have restored a backup that did not have those themes and plugins.