Add a streamlined easy-to-use Parental controls system

[grannovax]

The parental controls system I'm proposing is similar to the parental controls on Windows:
With it you can block websites, block access to certain apps, limit time use for certain apps, set time limits for the session as a whole. When the child user runs out of time, the parent should be able to enter the administrator password to extend screen time.

The lack of a parental controls system is a downside to Linux and doesn't really exist outside of GNOME, and the parental controls in GNOME aren't that great compared to Windows.

This addition would make Linux Mint more suitable for families