Fixes for Collage clips and the timeline

A small follow-up to 2.4.0, fixing four things reported in the first days.

Undo no longer deletes a Collage clip. Moving a Collage clip to the intro or outro and then pressing undo took the clip away entirely; it now goes back to where it was. The same fix applies to quick-added text, image and shape annotations.
Segments stay put when you zoom and scroll the timeline. Collage and Photo Motion segments could drift away from their position and disappear when placed in the outro.
The Collage slot follows the clip's duration. Changing Duration on an intro or outro Collage clip now resizes the slot on the video track with it.
Right-click menus on the timeline behave. Opening a segment's menu no longer leaves a dashed line following the cursor, and choosing a menu item no longer creates a stray segment.

Screenify Studio v2.4.1