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/widget/timelinewidget/view/timelineview.cpp

@@ -35,6 +35,7 @@
 #include "oakengine/preview.h"
 #include "oakengine/timeline.h"
 #include "oakengine/viewer.h"
+#include "playback/playbackcontroller.h"
 #include "panel/panelmanager.h"
 #include "panel/timeline/timeline.h"
 #include "widget/timelinewidget/cliphandle.h"
@@ -268,7 +269,7 @@ void TimelineView::mousePressEvent(QMouseEvent *event)
 		 it++) {
 		if (it.value().contains(scene_pos)) {
 			const Rational marker_in = marker_time_range(it.key()).in();
-			oakengine_viewer_set_playhead(get_viewer_node(),
+			PlaybackController::instance()->set_playhead(get_viewer_node(),
 			marker_in.numerator(),
 			marker_in.denominator());
 			break;