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/dialog/proxy/proxydialog.h

@@ -29,16 +29,17 @@
 #include "oakengine/proxy.h"
 #include "oakengine/videoparams.h"
 #include "oakengine/viewer.h"
-#include "node/project/footage/footage.h"
 #include "widget/slider/integerslider.h"
 
+struct OakEngineNode;
+
 namespace olive
 {
 
 class ProxyDialog : public QDialog {
 	Q_OBJECT
 public:
-	ProxyDialog(QWidget *parent, const QVector<Footage *> &footage = {});
+	ProxyDialog(QWidget *parent, const QVector<OakEngineNode *> &footage = {});
 
 	virtual void accept() override;
 
@@ -77,7 +78,7 @@ private:
 
 	void refresh_footage_list();
 
-	QVector<Footage *> footage_;
+	QVector<OakEngineNode *> footage_;
 
 	QTreeWidget *footage_tree_;