app: PlaybackController + playhead subscription migration (issues 0c, 1-4)

New app/playback/playbackcontroller.{h,cpp}: a lazily created
process-wide singleton whose set_playhead() forwards to
oakengine_viewer_set_playhead and re-broadcasts playhead_changed as a
plain Qt signal. Every app-side oakengine_viewer_set_playhead call site
(17 across viewer, timelinewidget, timelineview, import tool,
timebasedwidget, keyframecontrol, export dialog, mainwindow) now goes
through it.

Migrate the first four playhead subscribers off the raw C event
subscription / EngineEventBridge to the controller signal:
- TimeBasedView (issue 1)
- NodeParamViewWidgetBridge (issue 2)
- NodeParamViewKeyframeControl (issue 3)
- NodeParamViewConnectedLabel (issue 4)

Each stores a QMetaObject::Connection filtered on its viewer node and
disconnects on teardown; the viewer_sub_ members are gone. Plan docs
mark issues 0c and 1-4 as done.

Closes #30, closes #31, closes #32, closes #51, closes #52.

app/playback/playbackcontroller.cpp

@@ -0,0 +1,38 @@
+#include "playbackcontroller.h"
+
+namespace olive
+{
+
+PlaybackController *PlaybackController::instance_ = nullptr;
+
+PlaybackController *PlaybackController::instance()
+{
+	if (!instance_) {
+		instance_ = new PlaybackController();
+	}
+	return instance_;
+}
+
+PlaybackController::PlaybackController(QObject *parent)
+	: QObject(parent)
+{
+}
+
+void PlaybackController::set_playhead(OakEngineNode *viewer,
+									  const core::Rational &time)
+{
+	if (!viewer) {
+		return;
+	}
+	oakengine_viewer_set_playhead(viewer, time.numerator(),
+								  time.denominator());
+	emit playhead_changed(viewer, time);
+}
+
+void PlaybackController::set_playhead(OakEngineNode *viewer, int64_t num,
+									  int64_t den)
+{
+	set_playhead(viewer, core::Rational(num, den));
+}
+
+}