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/timelinewidget/timelinewidgetselections.cpp

@@ -31,7 +31,7 @@ void TimelineWidgetSelections::shift_time(const Rational &diff)
 	}
 }
 
-void TimelineWidgetSelections::shift_tracks(Track::Type type, int diff)
+void TimelineWidgetSelections::shift_tracks(TrackReference::Type type, int diff)
 {
 	TimelineWidgetSelections cached_selections;
 
@@ -51,7 +51,7 @@ void TimelineWidgetSelections::shift_tracks(Track::Type type, int diff)
 	// Then re-insert them with the diff applied
 	for (auto it = cached_selections.cbegin(); it != cached_selections.cend();
 		 it++) {
-		Track::Reference ref(it.key().type(), it.key().index() + diff);
+		TrackReference ref(it.key().type(), it.key().index() + diff);
 
 		this->insert(ref, it.value());
 	}
@@ -75,7 +75,7 @@ void TimelineWidgetSelections::subtract(
 	const TimelineWidgetSelections &selections)
 {
 	for (auto it = selections.cbegin(); it != selections.cend(); it++) {
-		const Track::Reference &track = it.key();
+		const TrackReference &track = it.key();
 		const TimeRangeList &their_list = it.value();
 
 		if (this->contains(track)) {