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/timeruler/seekablewidget.h

@@ -42,17 +42,17 @@ public:
 		return horizontalScrollBar()->value();
 	}
 
-	TimelineMarkerList *get_markers() const
+	OakEngineMarkerList *get_markers() const
 	{
 		return markers_;
 	}
-	TimelineWorkArea *get_work_area() const
+	OakEngineWorkarea *get_work_area() const
 	{
 		return workarea_;
 	}
 
-	void set_markers(TimelineMarkerList *markers);
-	void set_work_area(TimelineWorkArea *workarea);
+	void set_markers(OakEngineMarkerList *markers);
+	void set_work_area(OakEngineWorkarea *workarea);
 
 	virtual bool is_dragging_playhead() const override
 	{
@@ -83,7 +83,7 @@ public:
 		return !selection_manager_.get_selected_objects().empty();
 	}
 
-	const std::vector<TimelineMarker *> &get_selected_markers() const
+	const std::vector<OakEngineMarker *> &get_selected_markers() const
 	{
 		return selection_manager_.get_selected_objects();
 	}
@@ -146,8 +146,8 @@ private:
 
 	void commit_resize_handle();
 
-	TimelineMarkerList *markers_;
-	TimelineWorkArea *workarea_;
+	OakEngineMarkerList *markers_;
+	OakEngineWorkarea *workarea_;
 
 	int text_height_;
 
@@ -157,9 +157,9 @@ private:
 
 	bool ignore_next_focus_out_;
 
-	TimeBasedViewSelectionManager<TimelineMarker> selection_manager_;
+	TimeBasedViewSelectionManager<OakEngineMarker> selection_manager_;
 
-	QObject *resize_item_;
+	void *resize_item_;
 	ResizeMode resize_mode_;
 	TimeRange resize_item_range_;
 	QPointF resize_start_;