R8: finish app/ pure C ABI migration (P3-P9) and make OTIO required

- app/ no longer includes engine C++ headers nor holds engine C++ types:
  engine access goes through the oakengine C ABI plus C++ wrappers
  (oakutil/oaknode.h, oakutil/oakvideo.h) and app-local mirror types
  (tooltypes, trackreferencehandle, timelinecommonapp, keyframetypes,
  subtitleapp, serializedlayoutinfoapp, nodevaluehandle, sliderdisplaytypeapp)
- engine: new C ABI functions for block/track/clip/transition navigation
  and predicates, links, caches, waveform/playback, disk folder,
  sequence_track_list, node_free, footage_is_valid, block_get_track,
  get_brush; loadotio/saveotio ported to the current engine API
- OTIO is now a required dependency: CI and CD build it on every
  platform, FindOpenTimelineIO fixed for OTIO 0.16/0.19 (the old deps
  include requirement silently disabled OTIO everywhere), runtime
  libraries are bundled into packages and copied next to macOS binaries
  (oak_copy_otio_runtime)
- fix ProjectViewModel drag&drop mime read/write size mismatch (segfault)
- unify color label naming (k_olive -> "Oak") in the app-side mirror
- docs: OTIO required, FFmpeg minimum corrected to 6.0 (en/zh)
- gtest suite: 1925 passed, 0 failed

app/widget/viewer/vieweroutpututils.cpp

@@ -22,18 +22,14 @@
 
 namespace olive {
 
-VideoParams video_params_from_pod(const oak_video_params &pod)
+oak::VideoParams video_params_from_pod(const oak_video_params &pod)
 {
-	void *vp = oakengine_video_params_create(&pod);
-	VideoParams result = *static_cast<VideoParams *>(vp);
-	oakengine_video_params_free(vp);
-	return result;
+	return oak::VideoParams(pod);
 }
 
-VideoParams empty_video_params()
+oak::VideoParams empty_video_params()
 {
-	oak_video_params pod = {};
-	return video_params_from_pod(pod);
+	return oak::VideoParams();
 }
 
 Rational sequence_timebase(const void *sequence)
@@ -47,7 +43,43 @@ Rational sequence_timebase(const void *sequence)
 	return Rational(vp.time_base_num, vp.time_base_den);
 }
 
-VideoParams viewer_output_video_params(const void *viewer, int index)
+namespace {
+
+Rational viewer_rational(const void *viewer,
+						 int (*getter)(const OakEngineNode *, int64_t *,
+									   int64_t *))
+{
+	int64_t num = 0, den = 1;
+	if (getter(reinterpret_cast<const OakEngineNode *>(viewer), &num, &den) <
+		0) {
+		return Rational();
+	}
+	return Rational(num, den);
+}
+
+} // namespace
+
+Rational viewer_output_playhead(const void *viewer)
+{
+	return viewer_rational(viewer, oakengine_viewer_get_playhead);
+}
+
+Rational viewer_output_length(const void *viewer)
+{
+	return viewer_rational(viewer, oakengine_viewer_get_length);
+}
+
+Rational viewer_output_video_length(const void *viewer)
+{
+	return viewer_rational(viewer, oakengine_viewer_get_video_length);
+}
+
+Rational viewer_output_audio_length(const void *viewer)
+{
+	return viewer_rational(viewer, oakengine_viewer_get_audio_length);
+}
+
+oak::VideoParams viewer_output_video_params(const void *viewer, int index)
 {
 	oak_video_params vp;
 	if (oakengine_viewer_get_video_params(