timeline: use better system of identifying ruler focus

app/widget/timebased/timebasedwidget.cpp

@@ -741,7 +741,7 @@ void TimeBasedWidget::GoToOut()
 
 void TimeBasedWidget::DeleteSelected()
 {
-  if (ruler_->underMouse()) {
+  if (ruler_->HasItemsSelected()) {
     ruler_->DeleteSelected();
   }
 }

app/widget/timelinewidget/timelinewidget.cpp

@@ -441,7 +441,7 @@ void TimelineWidget::ReplaceBlocksWithGaps(const QVector<Block *> &blocks,
 
 void TimelineWidget::DeleteSelected(bool ripple)
 {
-  if (ruler()->hasFocus()) {
+  if (ruler()->HasItemsSelected()) {
     ruler()->DeleteSelected();
     return;
   }

app/widget/timeruler/seekablewidget.h

@@ -65,6 +65,11 @@ public:
 
   void SeekToScenePoint(qreal scene);
 
+  bool HasItemsSelected() const
+  {
+    return !selection_manager_.GetSelectedObjects().empty();
+  }
+
   const std::vector<TimelineMarker*> &GetSelectedMarkers() const
   {
     return selection_manager_.GetSelectedObjects();