Feature: Implement Cinematic Sequencer (Timeline Editor) for Cutscenes

[leoislam9090-ux]

This PR introduces the Cinematic Sequencer, a completely new and highly anticipated feature that allows users to create cutscenes, animations, and complex event sequences visually using a timeline editor inside GDevelop.

With this addition, users can manipulate cameras, animate object transforms, and trigger audio effortlessly along a timeline, translating directly into runtime events without having to build complex logic from scratch.

Key Features added:

1. Core C++ Architecture:
   • Introduced CinematicSequence, CinematicSequenceTrack, and CinematicSequenceKeyframe classes.
   • Fully integrated within the Project model for reliable saving and loading (Serialization).
   • SWIG bindings generated for JavaScript/newIDE interaction.
2. newIDE Integration:
   • Sequencer Editor GUI: A robust timeline editor component complete with Drag & Drop keyframe capabilities, multi-track support, and playback controls.
   • Live Scubbing & Preview: The timeline is synchronized directly with the SceneEditor, allowing users to preview their cutscenes live within the IDE editors.
   • Project Manager: Cinematic Sequences are now first-class citizens in the project manager tree with options to add, modify, delete, and copy.
3. Runtime Event Generator:
   • Developed CinematicSequencer extension containing new Actions/Conditions to play, pause, and control cinematic sequences directly from the Event Sheet.
   • Fully utilizes built-in timers and the Tween behavior for seamless interpolation between keyframes.

Testing Performed

• Verified successful C++ core compilation with the new files.
• Confirmed saving and reloading of .json project files retain complete sequence data.
• Tested UI robustness, including switching tabs between layouts and sequences.
• Tested the connection between the Sequence Editor and the Scene Editor (changing timeframe updates the scene).
• Verified runtime playback triggers correctly when initiated via Event Sheet execution.

How to check this out

1. Open the GDevelop IDE and look at the Project Manager panel.
2. Under "External layouts", you will find the new "Cinematic Sequences" folder.
3. Create a new sequence and double-click to open it.
4. Try adding new tracks, keyframes, and scrub the timeline to observe changes!