miscellaneous other changes to make audio rendering work

Many changes were made throughout the codebase to support audio, these are
most of the small changes necessary.

The audio support still is not perfect. I still need to write in resampling
support. After that it should work correctly with all audio types.

app/node/block/block.cpp

@@ -110,6 +110,7 @@ QVariant Block::Value(NodeOutput *output)
 void Block::EdgeAddedSlot(NodeEdgePtr edge)
 {
   if (edge->input() == previous_input()) {
+    // FIXME: No protection for if this connection is not a node
     static_cast<Block*>(edge->output()->parent())->next_ = this;
 
     // The blocks surrounding this one have changed, we need to Refresh()
@@ -123,6 +124,7 @@ void Block::EdgeAddedSlot(NodeEdgePtr edge)
 void Block::EdgeRemovedSlot(NodeEdgePtr edge)
 {
   if (edge->input() == previous_input()) {
+    // FIXME: No protection for if this connection is not a node
     static_cast<Block*>(edge->output()->parent())->next_ = nullptr;
 
     // The blocks surrounding this one have changed, we need to Refresh()