Rename only visible to only selected

This MR renames the option to only track people selected to be visible from only visible to only selected for differentiation from show only visible and to make it clearer that it really depends on the selection i.e. not every visible person gets tracked.

I don't remember whether Issue #90 (closed) had other problems as well. So I won't close it via this MR before further discussion.

Related to #90 (closed)