Modernize Window/CvTrackbar; delete DisposableObject (closes #1941)

[shimat]

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Final slice of #1941 (part of #1938). Stacked on #1970 (review/merge order: #1968 → #1969 → #1970 → this). Migrates the last two DisposableObject subclasses off it and removes DisposableObject entirely — the goal of #1941.

Decision (with @shimat): keep the OO Window/CvTrackbar API (it has value as an object form of OpenCVs name-keyed highgui), but modernize it. OpenCV 5s highgui design is unchanged (name-based namedWindow/imshow/setMouseCallback/createTrackbar, with MouseCallback/TrackbarCallback typedefs), so the wrappers premise is stable.

Centralized callback lifetime (also fixes a latent crash)

OpenCV highgui stores raw function pointers per window name and invokes them later from its own UI loop, so the managed delegates must stay alive until the window is destroyed. Cv2.SetMouseCallback / Cv2.CreateTrackbar previously passed the delegate straight to native without rooting it — the GC could collect a delegate OpenCV still calls → crash.

Added a name-keyed registry in Cv2 that roots mouse/trackbar delegates and clears them on DestroyWindow / DestroyAllWindows. This mirrors OpenCVs own name-keyed, process-global model and is now the single place these delegates are rooted.

Window

• Now sealed class : IDisposable (off DisposableObject); no finalizer.
• Removed dead code: the MouseCallback property + ScopedGCHandle machinery (its setter was never called anywhere) and the NotImplementedException DisplayOverlay/DisplayStatusBar stubs.
• Dispose routes through Cv2.DestroyWindow, which also forgets the windows callbacks.

CvTrackbar

• Now a plain sealed class (no IDisposable). It previously held two GCHandles solely to keep the callback delegates alive — the Cv2 registry now does that. Creation routes through Cv2.CreateTrackbar; no GCHandle, no GetFunctionPointerForDelegate.

Cleanup

• Deleted DisposableObject (no remaining subclasses) and the now-unused ScopedGCHandle helper.

Notes

• Window.GetWindowByName and the static Dictionary<string, Window> registry are kept as-is (still public API). They remain a minor global-state smell but are out of scope here; can be revisited separately.
• Managed + test projects build clean. highgui runtime behavior validated on CI.
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