Improve timeline undo performance #1897

This pull request significantly improves the undo/redo system for timeline editing commands by making undo operations more efficient and context-aware. The changes focus on capturing only the affected tracks or clips for undo, rather than restoring entire tracks unnecessarily. This results in faster, more reliable undos and better user experience. Additionally, command descriptions are now copied to UndoHelper to show on a progress dialogs when things are slow. Things are usually slow on big projects especially when Ripple All Tracks is on.

Undo/Redo System Improvements

• Most timeline commands now call m_undoHelper.recordBeforeState() with a set of affected track indices, instead of always restoring whole tracks. This enables more efficient, fine-grained undo operations.




• For commands that can affect multiple tracks (such as ripple edits or move/align commands), logic was added to determine the precise scope of affected tracks and fall back to whole-track restores only when necessary (e.g., ripple-all-tracks is enabled).

Here's the complete picture of every timeline command's undo strategy on the branch now.

Fine-grained (fast per-clip undo — no RestoreTracks)

Use UndoHelper with NoHints/SkipXML:

| Command | Notes |
|---|---|
| AppendCommand | NoHints |
| OverwriteCommand | NoHints (snapshots the replaced/split span) |
| LiftCommand | NoHints (snapshots lifted clip) |
| RemoveCommand | NoHints when not ripple-all |
| MergeCommand | NoHints |
| SplitCommand | NoHints (snapshots split clip) |
| InsertCommand | NoHints when not ripple-all (snapshots split clip) |
| AddTransitionCommand | NoHints when not ripple-all (snapshots mixed clips) |
| TrimClipInCommand | NoHints when not ripple |
| TrimClipOutCommand | SkipXML when not ripple |
| UpdateCommand | NoHints |
| DetachAudioCommand | NoHints |
| ReplaceCommand | NoHints |

Still RestoreTracks (whole-track rebuild)

These are the genuinely track-wide or cross-track cases where fine-grained doesn't apply:

| Command | When | Why it needs it |
|---|---|---|
| MoveClipCommand | always | can move across tracks / reorder |
| RemoveTrackCommand | always | the whole track is cleared |
| AlignClipsCommand | always | repositions many clips (lift+overwrite loop) |
| InsertCommand | ripple-all-tracks | shifts clips on other tracks |
| RemoveCommand | ripple-all-tracks | shifts other tracks |
| AddTransitionCommand | ripple-all-tracks | shifts other tracks |
| TrimClipInCommand | ripple | ripple shifts the track (and others if ripple-all) |

Don't use UndoHelper at all (own inverse logic)

GroupCommand, UngroupCommand, NameTrackCommand, MuteTrackCommand, HideTrackCommand, CompositeTrackCommand, LockTrackCommand, FadeInCommand, FadeOutCommand, TrimTransitionInCommand, TrimTransitionOutCommand, ResizeTransitionCommand, AddTransitionByTrimInCommand, AddTransitionByTrimOutCommand, RemoveTransitionByTrimInCommand, RemoveTransitionByTrimOutCommand, AddTrackCommand, InsertTrackCommand, MoveTrackCommand, ChangeBlendModeCommand, ChangeTransitionPropertyCommand, ApplyFiltersCommand, ChangeGainCommand.

Progress Dialog

• All timeline commands now propagate their undo/redo text label via m_undoHelper.setText(text()) to show descriptions in the progress dialogs.

Improved Undo Data Capture

• For commands that split, overwrite, or otherwise modify clips, the system now snapshots only the affected clips’ XML, enabling cheap and accurate restoration without parsing the entire track.

Refactoring for Move/Align Commands

• The MoveClipCommand and AlignClipsCommand now defer the call to recordBeforeState() until the redo step, after the set of affected clips/tracks is known, further optimizing the undo scope.