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/node/block/transition/transition.cpp

@@ -0,0 +1,62 @@
+#include "transition.h"
+
+TransitionBlock::TransitionBlock()
+{
+  out_block_input_ = new NodeInput("out_block_in");
+  AddInput(out_block_input_);
+
+  in_block_input_ = new NodeInput("in_block_in");
+  AddInput(in_block_input_);
+}
+
+Block::Type TransitionBlock::type() const
+{
+  return kTransition;
+}
+
+NodeInput *TransitionBlock::out_block_input() const
+{
+  return out_block_input_;
+}
+
+NodeInput *TransitionBlock::in_block_input() const
+{
+  return in_block_input_;
+}
+
+void TransitionBlock::Retranslate()
+{
+  out_block_input_->set_name(tr("From"));
+  in_block_input_->set_name(tr("To"));
+}
+
+void TransitionBlock::set_length(const rational &length)
+{
+  Q_UNUSED(length)
+  qCritical() << "Set length is not permitted on a transition";
+  abort();
+}
+
+void TransitionBlock::set_length_and_media_in(const rational &length)
+{
+  Q_UNUSED(length)
+  qCritical() << "Set length and media in is not permitted on a transition";
+  abort();
+}
+
+void TransitionBlock::set_in_offset(const rational &in_offset)
+{
+  in_offset_ = in_offset;
+  RecalculateLength();
+}
+
+void TransitionBlock::set_out_offset(const rational &out_offset)
+{
+  out_offset_ = out_offset;
+  RecalculateLength();
+}
+
+void TransitionBlock::RecalculateLength()
+{
+  Block::set_length(in_offset_ + out_offset_);
+}