Daily usage limit

[AllanChain]

See also #3. Daily usage limit is useful for parental control.

We should design a way to tell kids that they have used the computer too long without annoying them.

This feature needs:

• Persistent storage of daily usage
• A window to indicate daily usage limit reached (and probably before that)

[wolftune]

I think like the other breaks, there's some amount of time after the warning (which can just be the blinking window) when it forces full-screen saying "daily limit" just instead of "time for a small break". At least that's how WorkRave does it. But this makes the ability to shut-down from the break screen more important.

I'm not sure what is best, except that it needs to be easy enough for kids to really be pushed to not ignore the limit and for parents to glance from a distance and see that it has been hit.