make nodes more const friendly and prepare their functions for stateless work

Since the nodes won't be holding any rendering data themselves in this system,
we may as well enforce some level of non-write access by setting all the
functions to const. They were already const-friendly, they just weren't
labelled as such.

app/node/blend/alphaover/alphaover.cpp

@@ -25,27 +25,27 @@ AlphaOverBlend::AlphaOverBlend()
 
 }
 
-Node *AlphaOverBlend::copy()
+Node *AlphaOverBlend::copy() const
 {
   return new AlphaOverBlend();
 }
 
-QString AlphaOverBlend::Name()
+QString AlphaOverBlend::Name() const
 {
   return tr("Alpha Over");
 }
 
-QString AlphaOverBlend::id()
+QString AlphaOverBlend::id() const
 {
   return "org.olivevideoeditor.Olive.alphaoverblend";
 }
 
-QString AlphaOverBlend::Description()
+QString AlphaOverBlend::Description() const
 {
   return tr("A blending node that composites one texture over another using its alpha channel.");
 }
 
-QString AlphaOverBlend::Code(NodeOutput *output)
+QString AlphaOverBlend::Code(NodeOutput *output) const
 {
   if (output == texture_output()) {
     return "#version 110"