Improve Analyze zoom-in.

Adjusts how zoom-in works. Previously it always zoomed in to the center of the display. This doesn't make sense if you've set a cursor, and, e.g. are interested in that part of the display. Now, if there's a cursor, it zooms in to the cursor. If the display is tied to the end of the current session, it keeps it that way.

To test, open Analyze with some data, set a cursor somewhere and zoom-in. The cursor position should be in the middle of the display. Zoom in again, should likewise zoom further centered on the cursor.