Add "Add Markers at Gaps" feature to Timeline menu
This MR implements the feature requested in bug 352409, which automatically places guide markers at the start of every gap between video clips in the timeline.
A new action "Add Markers at Gaps" is added to the Timeline menu. When triggered, it scans all video tracks, finds gaps between consecutive clips, and places a guide marker at the start of each gap. Duplicate positions across multiple tracks are deduplicated, audio-only tracks are ignored, and if no gaps are found an info message "No gaps found in timeline" is shown.
I am opening this MR as draft to get feedback on the UI approach, specifically wheter this should be a standalone menu action or part of a larger "Check project" dialog as discussed in the bug report. I think, having it as a standalone Timeline menu action feels more intuitive, since users would want to find and fix gaps during editing, not right before the rendering.