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/generator/solid/solid.cpp

@@ -31,37 +31,37 @@ SolidGenerator::SolidGenerator() :
   AddParameter(texture_output_);
 }
 
-Node *SolidGenerator::copy()
+Node *SolidGenerator::copy() const
 {
   return new SolidGenerator();
 }
 
-QString SolidGenerator::Name()
+QString SolidGenerator::Name() const
 {
   return tr("Solid");
 }
 
-QString SolidGenerator::id()
+QString SolidGenerator::id() const
 {
   return "org.olivevideoeditor.Olive.solidgenerator";
 }
 
-QString SolidGenerator::Category()
+QString SolidGenerator::Category() const
 {
   return tr("Generator");
 }
 
-QString SolidGenerator::Description()
+QString SolidGenerator::Description() const
 {
   return tr("Generate a solid color.");
 }
 
-NodeOutput *SolidGenerator::texture_output()
+NodeOutput *SolidGenerator::texture_output() const
 {
   return texture_output_;
 }
 
-QString SolidGenerator::Code(NodeOutput *output)
+QString SolidGenerator::Code(NodeOutput *output) const
 {
   if (output == texture_output()) {
     // FIXME: Not color managed

app/node/generator/solid/solid.h

@@ -34,16 +34,16 @@ class SolidGenerator : public Node
 public:
   SolidGenerator();
 
-  virtual Node* copy() override;
+  virtual Node* copy() const override;
 
-  virtual QString Name() override;
-  virtual QString id() override;
-  virtual QString Category() override;
-  virtual QString Description() override;
+  virtual QString Name() const override;
+  virtual QString id() const override;
+  virtual QString Category() const override;
+  virtual QString Description() const override;
 
-  NodeOutput* texture_output();
+  NodeOutput* texture_output() const;
 
-  virtual QString Code(NodeOutput* output) override;
+  virtual QString Code(NodeOutput* output) const override;
 
 private:
   NodeInput* color_input_;