Allow manual sorting of perspectives

[sipraga]

Users can select a manual sorting method and then rearrange the list of perspectives using newly added functions pers-sorting-move-left/right.

[gcv]

Thanks, this looks like a useful feature. Unfortunately, the implementation does not work for me.

First, the change to the perspective struct adds a field called sorted-time, but all the code references persp-sorting-time.

Second, even if I change persp-sorting-time to persp-sorted-time, calls to (persp-names) still fail.