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

include/audacity/ImporterInterface.h

@@ -72,7 +72,7 @@ public:
    // Get a list of extensions this plugin expects to be able to
    // import.  If a filename matches any of these extensions,
    // this importer will get first dibs on importing it.
-   virtual wxArrayString GetSupportedExtensions() = 0;
+   virtual FileExtensions GetSupportedExtensions() = 0;
    virtual bool SupportsExtension(const wxString & extension) = 0;
 
    // Create the client that will be used to import a file.

include/audacity/ModuleInterface.h

@@ -84,7 +84,7 @@ public:
    // This function returns nonempty only when that is the case, and lists
    // the possible extensions of such files (an empty string in a nonempty
    // array means any file is a candidate).
-   virtual wxArrayString GetFileExtensions() = 0;
+   virtual FileExtensions GetFileExtensions() = 0;
 
    // Returns empty, or else, where to copy a plug-in file or bundle.
    // Drag-and-drop is supported only if GetFileExtensions() returns nonempty and

include/audacity/Types.h

@@ -67,6 +67,12 @@ using PluginPaths = std::vector< PluginPath >;
 using RegistryPath = wxString;
 using RegistryPaths = std::vector< RegistryPath >;
 
+class wxArrayStringEx;
+
+// File extensions, not including any leading dot
+using FileExtension = wxString;
+using FileExtensions = wxArrayStringEx;
+
 // ----------------------------------------------------------------------------
 // A native 64-bit integer...used when referring to any number of samples
 // ----------------------------------------------------------------------------