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/tool/import.cpp

@@ -36,6 +36,7 @@
 #include "oakengine/timeline.h"
 #include "oakengine/undo.h"
 #include "oakengine/viewer.h"
+#include "playback/playbackcontroller.h"
 #include "oakengine/project.h"
 #include "widget/timelinewidget/cliphandle.h"
 #include "window/mainwindow/mainwindow.h"
@@ -252,7 +253,7 @@ void ImportTool::place_at(const DraggedFootageData &footage,
 	drop_ghosts(insert, command);
 
 	if (jump_to_end) {
-		oakengine_viewer_set_playhead(
+		PlaybackController::instance()->set_playhead(
 			reinterpret_cast<OakEngineNode *>(this->sequence()),
 			max.numerator(), max.denominator());
 	}