Allow user-specified size limit for timeshift buffer

[prl]

If a T series is left on without changing channels for some time, the timeshift buffer grows without limit.

It would be useful (especially for users with small HDDs) to allow the maximum size of the timeshift buffer to be limited.

This suggestion was prompted by a report of a user with a 128GB SSD having recordings fail because the timeshift buffer consumed a large amout of the available space.

The buffer size could be checked as each new event starts.