Use type aliases FileExtension, FileExtensions...

... for wxString and wxArrayStringEx, holding file extensions (without dot);
to be replaced later with different types

(not yet using std::vector, becase of some uses of wxArrayString::Index with
two arguments)

And simplify some constructions of arrays

src/effects/vamp/LoadVamp.h

@@ -41,7 +41,7 @@ public:
    bool Initialize() override;
    void Terminate() override;
 
-   wxArrayString GetFileExtensions() override { return {}; }
+   FileExtensions GetFileExtensions() override { return {}; }
    wxString InstallPath() override { return {}; }
 
    bool AutoRegisterPlugins(PluginManagerInterface & pm) override;