Check footage in use before deleteing

Initial very hacky implementation that checks if the footgae is in
use before deleting it and makes sure all refernences to it are
properly cleaned up.

Also added IsMedia() function to Node to simplify things a bit.

app/node/input/media/media.cpp

@@ -50,6 +50,11 @@ void MediaInput::SetFootage(StreamPtr f)
   footage_input_->set_standard_value(QVariant::fromValue(f));
 }
 
+bool MediaInput::IsMedia() const
+{
+  return true;
+}
+
 void MediaInput::Retranslate()
 {
   footage_input_->set_name(tr("Footage"));