removed all dependence on NodeOutputs

If Nodes only have the one output, we don't need to do so much differentiation
between them. Previous iteration used outputs as like a distinct function
within a Node (e.g. length output would return one result, buffer output would
produce a different result - each run different code to produce their results).
Now in this iteration, it's more accurate to say a Node is just one function
(which seems more appropriate for a node system anyway).

app/widget/timelinewidget/timelinewidget.cpp

@@ -315,7 +315,7 @@ void TimelineWidget::SplitAtPlayhead()
   foreach (TrackOutput* track, timeline_node_->Tracks()) {
     Block* b = track->BlockContainingTime(playhead_time);
 
-    if (b != nullptr) {
+    if (b) {
       bool selected = false;
 
       // See if this block is selected