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/panel/timebased/timebased.cpp

@@ -21,6 +21,8 @@
 
 #include "timebased.h"
 
+#include "oakutil/oaknode.h"
+
 namespace olive
 {
 
@@ -116,7 +118,7 @@ void TimeBasedPanel::shuttle_right()
 	emit shuttle_right_requested();
 }
 
-void TimeBasedPanel::connect_viewer_node(ViewerOutput *node)
+void TimeBasedPanel::connect_viewer_node(OakEngineNode *node)
 {
 	widget_->connect_viewer_node(node);
 }
@@ -140,8 +142,8 @@ void TimeBasedPanel::set_time_based_widget(TimeBasedWidget *widget)
 
 void TimeBasedPanel::retranslate()
 {
-	if (get_time_based_widget()->get_connected_node()) {
-		set_subtitle(get_time_based_widget()->get_connected_node()->get_label());
+	if (get_connected_viewer()) {
+		set_subtitle(oak::Node(get_connected_viewer()).get_label());
 	} else {
 		set_subtitle(tr("(none)"));
 	}