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/block/gap/gap.cpp

@@ -24,7 +24,7 @@ GapBlock::GapBlock()
 {
 }
 
-Node *GapBlock::copy()
+Node *GapBlock::copy() const
 {
   GapBlock* c = new GapBlock();
 
@@ -33,22 +33,22 @@ Node *GapBlock::copy()
   return c;
 }
 
-Block::Type GapBlock::type()
+Block::Type GapBlock::type() const
 {
   return kGap;
 }
 
-QString GapBlock::Name()
+QString GapBlock::Name() const
 {
   return tr("Gap");
 }
 
-QString GapBlock::id()
+QString GapBlock::id() const
 {
   return "org.olivevideoeditor.Olive.gap";
 }
 
-QString GapBlock::Description()
+QString GapBlock::Description() const
 {
   return tr("A time-based node that represents an empty space.");
 }

app/node/block/gap/gap.h

@@ -32,13 +32,13 @@ class GapBlock : public Block
 public:
   GapBlock();
 
-  virtual Node * copy() override;
+  virtual Node * copy() const override;
 
-  virtual Type type() override;
+  virtual Type type() const override;
 
-  virtual QString Name() override;
-  virtual QString id() override;
-  virtual QString Description() override;
+  virtual QString Name() const override;
+  virtual QString id() const override;
+  virtual QString Description() const override;
 
 private: