implemented deleting clips in the timeline

Also fixed bugs in the Node::GetExclusiveDependencies function and in the
NodeInputArray::RemoveAt function to make this functionality work.

app/panel/timeline/timeline.cpp

@@ -114,6 +114,11 @@ void TimelinePanel::GoToNextCut()
   timeline_widget_->GoToNextCut();
 }
 
+void TimelinePanel::DeleteSelected()
+{
+  timeline_widget_->DeleteSelected();
+}
+
 void TimelinePanel::changeEvent(QEvent *e)
 {
   if (e->type() == QEvent::LanguageChange) {