Add Spine 4.3 object, timeline sequencer, and desktop window controls

[mousekkkss]

Summary

This pull request adds a set of desktop/game-production features on top of the current upstream GDevelop codebase:

• Adds Spine 4.3 object/runtime support for projects that need newer Spine assets.
• Adds timeline sequencer support for authoring and controlling timeline-based behavior.
• Adds desktop pet style window controls in the Advanced Window extension.
• Adds native transparent window/background support for desktop previews and Electron exports.
• Adds event-driven frameless game window support that applies to both local preview windows and exported desktop Electron games.
• Adds editor/export plumbing so desktop window options can be detected from project events and propagated to preview, manual desktop export, and online desktop export flows.
• Adds GDevelop MCP/editor bridge work so external tools can inspect and modify project/editor state more directly.
• Keeps the MCP/editor bridge opt-in so the editor no longer attempts local WebSocket connections unless a bridge URL or bridge flag is explicitly provided.
• Fixes a nested button structure in the object creation list to avoid React DOM nesting warnings.

Desktop pet and hidden background changes

The Advanced Window extension now includes desktop-window controls intended for desktop pet style games and overlays:

• transparent native window background support;
• transparent runtime/game canvas background support;
• no-shadow/frameless window behavior;
• optional always-on-top behavior;
• taskbar visibility control;
• mouse click-through behavior;
• menu bar visibility control;
• docking helpers for placing the window near screen edges/corners.

These changes make it possible to create desktop pet or overlay games where the normal rectangular game background is hidden and only the visible game objects remain on screen.

Frameless window behavior

The previous preview-only frameless action has been updated into a game-window action:

• the action is now labeled as a frameless game window action rather than a preview-only action;
• local desktop previews use the event setting before creating the preview window;
• exported Electron desktop games use the same event setting before creating the packaged game window;
• manual desktop export and online desktop export both merge explicit export window settings with event-derived window settings;
• if any layout or external event sheet enables the frameless game window action, desktop export enables Electron frameless mode automatically.

Localization

The new Advanced Window action strings are wired through the extension translation system and include updated English, Simplified Chinese, and Traditional Chinese compiled extension message entries.

Validation

Validated locally with:

• Advanced Window extension reload/import into the editor resources.
• ESLint on the touched export/preview files.
• Flow focus-check on the touched export/preview files.
• Full Flow check for the editor app.
• Electron main/preview script syntax checks.
• Full Windows Electron app build with the updated editor bundle.
• Build output language chunk check confirming the new Chinese strings are present and no question-mark mojibake remains.
• MCP bridge opt-in behavior checked so the editor stays quiet when the MCP server is not running.
• Object creation list DOM nesting warning fixed by removing the button-inside-button structure.

[mousekkkss]

Note for reviewers about the Semaphore fast tests:

The previous red ci/semaphoreci/pr: Fast tests (not building GDevelop.js - can have false negatives) run was traced to the downloaded libGD.js artifact used by CI, not to Flow/test failures in the Timeline/Spine43 implementation itself.

What happened in the failing run:

../public/libGD.js:22:1 - error TS1109: Expression expected.
Found 11 errors in the same file, starting at: ../public/libGD.js:22

The newIDE tests job failed for the same reason while loading newIDE/app/node_modules/libGD.js-for-tests-only/index.js, also at line 22.

I pushed follow-up commit a3a198b to harden newIDE/app/scripts/import-libGD.js for PR CI:

• resolve detached parent commits back to their remote branch when available;
• fall back to master for merge-parent refs used by PR CI;
• run node --check on downloaded libGD.js before copying it into the test dependency, so a bad downloaded artifact is rejected and the script can try the next candidate;
• document the generated libGD.js import boundary used by the documentation scripts.

Local verification on the PR branch passes:

npm run check-script-types
npm run check-format
git diff --check
newIDE tests: 89 test suites passed, 664 tests passed, 1 skipped

The new Semaphore workflow for this commit passed: https://gdevelop.semaphoreci.com/workflows/858b7b1e-9e7a-437f-985c-4b6d7741c79f?pipeline_id=ffd88da3-d9ea-404c-a4bf-b4abb55dcc40

Travis is still queued at the time of writing.