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/timeline/timelinecoordinate.cpp

@@ -25,19 +25,19 @@ namespace olive
 {
 
 TimelineCoordinate::TimelineCoordinate()
-	: track_(Track::k_none, 0)
+	: track_(TrackReference::k_none, 0)
 {
 }
 
 TimelineCoordinate::TimelineCoordinate(const Rational &frame,
-									   const Track::Reference &track)
+									   const TrackReference &track)
 	: frame_(frame)
 	, track_(track)
 {
 }
 
 TimelineCoordinate::TimelineCoordinate(const Rational &frame,
-									   const Track::Type &track_type,
+									   const TrackReference::Type &track_type,
 									   const int &track_index)
 	: frame_(frame)
 	, track_(track_type, track_index)
@@ -49,7 +49,7 @@ const Rational &TimelineCoordinate::get_frame() const
 	return frame_;
 }
 
-const Track::Reference &TimelineCoordinate::get_track() const
+const TrackReference &TimelineCoordinate::get_track() const
 {
 	return track_;
 }
@@ -59,7 +59,7 @@ void TimelineCoordinate::set_frame(const Rational &frame)
 	frame_ = frame;
 }
 
-void TimelineCoordinate::set_track(const Track::Reference &track)
+void TimelineCoordinate::set_track(const TrackReference &track)
 {
 	track_ = track;
 }