Where did that clip go?

Fixes:

• Adding a suggested clip appeared to do nothing on longer projects. The clip was in fact being added correctly — at the end of the main track, which on a long timeline is well past what you can see. Lucy now moves the playhead to the new clip and selects it, and the message tells you where it landed.
• A suggestion with no length could produce an invisible, zero-length clip. Those are skipped now, and if nothing could be added you get an error instead of a success message.