Replace virtual with override wherever possible; eliminate needless virtual...

... for functions in final classes.

override is like const -- it's not necessary, but it helps the compiler to
catch mistakes.

There may be some overriding functions not explicitly declared virtual and I did
not identify such cases, in which I might also add override.

src/prefs/SpectrogramSettings.cpp

@@ -146,7 +146,7 @@ const wxArrayString &SpectrogramSettings::GetScaleNames()
 {
    class ScaleNamesArray final : public TranslatableStringArray
    {
-      virtual void Populate()
+      void Populate() override
       {
          // Keep in correspondence with enum SpectrogramSettings::ScaleType:
          mContents.Add(_("Linear"));
@@ -171,7 +171,7 @@ const wxArrayString &SpectrogramSettings::GetAlgorithmNames()
 {
    class AlgorithmNamesArray final : public TranslatableStringArray
    {
-      virtual void Populate()
+      void Populate() override
       {
          // Keep in correspondence with enum SpectrogramSettings::Algorithm:
          mContents.Add(_("Frequencies"));