allow gaps and transitions to be selected in the timeline

This will be useful for various reasons, however the behavior of selecting
gaps and transitions will need to be regulated harder than clips in the near
future.

app/widget/timelinewidget/view/timelineviewblockitem.cpp

@@ -46,7 +46,10 @@ void TimelineViewBlockItem::SetBlock(Block *block)
 {
   block_ = block;
 
-  setFlag(QGraphicsItem::ItemIsSelectable, block_->type() == Block::kClip);
+  setFlag(QGraphicsItem::ItemIsSelectable,
+          block_->type() == Block::kClip
+          || block_->type() == Block::kGap
+          || block_->type() == Block::kTransition);
 
   UpdateRect();
 }