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();

app/widget/timelinewidget/view/timelineviewblockitem.cpp

@@ -79,6 +79,7 @@ void TimelineViewBlockItem::paint(QPainter *painter, const QStyleOptionGraphicsI
 
   switch (block_->type()) {
   case Block::kClip:
+  case Block::kTransition: // FIXME: Temporary, the transition should have its own UI representation at some point
     /*QLinearGradient grad;
     grad.setStart(0, rect().top());
     grad.setFinalStop(0, rect().bottom());