started base transition type

Like the base "Block" type, the "Transition" type will not be a node used on its
own, it will have to be derived. This type defines some basic behavior for a
transition (e.g. how much it overlaps its "from" and "to" blocks).

app/widget/timelinewidget/timelinewidget.cpp

@@ -573,6 +573,7 @@ void TimelineWidget::AddBlock(Block *block, TrackReference track)
 {
   switch (block->type()) {
   case Block::kClip:
+  case Block::kTransition:
   case Block::kGap:
   {
     TimelineViewBlockItem* item = new TimelineViewBlockItem();