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/widget/timelinewidget/tool/tool.cpp

@@ -142,7 +142,7 @@ void TimelineTool::get_ghost_data(Rational *earliest_point,
 	}
 }
 
-void TimelineTool::insert_gaps_at_ghost_destination(olive::MultiUndoCommand *command)
+void TimelineTool::insert_gaps_at_ghost_destination(void *command)
 {
 	Rational earliest_point, latest_point;