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/import/ImportPCM.cpp

@@ -73,9 +73,8 @@ class PCMImportPlugin final : public ImportPlugin
 {
 public:
    PCMImportPlugin()
-   :  ImportPlugin(wxArrayString())
+   :  ImportPlugin(sf_get_all_extensions())
    {
-      mExtensions = sf_get_all_extensions();
    }
 
    ~PCMImportPlugin() { }