serialize rationals correctly and move block name to a node input

Since rationals aren't a known Qt format, the QVariant container can't
automatically convert them to and from strings (for XML serialization). We have
to hijack these functions and do the conversion manually for those types.

Also moved the block name type to a node input, which means it's serialized
and copied by default (I'm not sure why it wasn't already like this).

app/project/item/sequence/sequence.cpp

@@ -141,7 +141,9 @@ void Sequence::Load(QXmlStreamReader *reader)
 
   // Ensure this and all children are in the main thread
   // (FIXME: Weird place for this? This should probably be in ProjectLoadManager somehow)
-  moveToThread(qApp->thread());
+  if (thread() != qApp->thread()) {
+    moveToThread(qApp->thread());
+  }
 }
 
 void Sequence::Save(QXmlStreamWriter *writer) const