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/dialog/export/export.cpp

@@ -47,6 +47,7 @@
 #include "oakengine/encoding.h"
 #include "oakengine/timeline.h"
 #include "oakengine/viewer.h"
+#include "playback/playbackcontroller.h"
 #include "ui/icons/icons.h"
 #include "widget/timeruler/timeruler.h"
 #include "common/configwrapper.h"
@@ -319,7 +320,7 @@ ExportDialog::ExportDialog(OakEngineNode *viewer_node, bool stills_only_mode,
 		this);
 	connect(video_tab_, &ExportVideoTab::time_changed, this,
 			[viewer_node](const Rational &time) {
-				oakengine_viewer_set_playhead(
+				PlaybackController::instance()->set_playhead(
 					viewer_node,
 					time.numerator(), time.denominator());
 			});