reimplemented "add" tool

Tool for adding empty/non-media clips in the timeline. Also basis of the
transition tool.

app/widget/timelinewidget/tool/transition.cpp

@@ -0,0 +1,29 @@
+#include "widget/timelinewidget/timelinewidget.h"
+
+TimelineWidget::TransitionTool::TransitionTool(TimelineWidget *parent) :
+  Tool(parent)
+{
+}
+
+void TimelineWidget::TransitionTool::MousePress(TimelineViewMouseEvent *event)
+{
+  TimelineViewBlockItem* item = GetItemAtScenePos(event->GetCoordinates());
+
+  bool selectable_item = (item != nullptr
+      && item->flags() & QGraphicsItem::ItemIsSelectable
+      && !parent()->GetTrackFromReference(item->Track())->IsLocked());
+
+  if (!selectable_item) {
+    return;
+  }
+}
+
+void TimelineWidget::TransitionTool::MouseMove(TimelineViewMouseEvent *event)
+{
+  Q_UNUSED(event)
+}
+
+void TimelineWidget::TransitionTool::MouseRelease(TimelineViewMouseEvent *event)
+{
+  Q_UNUSED(event)
+}