Fix negative indices in MonitoredFocusList

This is a fix for #73.

It turned out only MonitoredFocusList.__delitem__() was broken. This PR fixes it and adds a few doctest's that verify support of negative (slice) indices.

Please especially do check if you agree with ba1c39341504922d4a7af8453d929de71ad86c13 which deletes the bodies and doctest's of 2 (deprecated) methods and turns them into simple wrappers.