app: migrate all engine access to the C ABI facade (nm U _ZN5olive = 0)

Every app module now reaches liboakengine exclusively through
oakengine_* C calls, EngineEventBridge subscriptions and app-side
handle headers (cliphandle/keyframehandle/nodevaluehandle/oakvaluehelper).
Direct C++ command construction, engine signal connect()s, and engine
type usage in MOC-visible signatures are gone: 557 -> 0 undefined
olive:: symbols in oak-editor.

app/dialog/speedduration/speeddurationdialog.h

@@ -26,14 +26,13 @@
 #include <QComboBox>
 #include <QDialog>
 
-#include "node/block/clip/clip.h"
 #include "node/block/gap/gap.h"
-#include "undo/undocommand.h"
 #include "widget/slider/floatslider.h"
 #include "widget/slider/rationalslider.h"
 
-namespace olive
-{
+namespace olive {
+
+class ClipBlock;
 
 class SpeedDurationDialog : public QDialog {
 	Q_OBJECT