Break dependency cycle introduced at 50f3321...

... This only cuts and pastes some functions from FileNames to another new file
and namespace, and changes namespace qualifiers where they are called.

The cycle resulted because of the need to include widgets/ErrorDialog.h

src/FileNames.h

@@ -65,12 +65,6 @@ namespace FileNames
    bool HardLinkFile( const FilePath& file1, const FilePath& file2);
 
    wxString MkDir(const wxString &Str);
-   wxString TempDir();
-   void ResetTempDir();
-
-   const FilePath &DefaultTempDir();
-   void SetDefaultTempDir( const FilePath &tempDir );
-   bool IsTempDirectoryNameOK( const FilePath & Name );
 
    bool IsMidi(const FilePath &fName);
 
@@ -205,17 +199,11 @@ namespace FileNames
    wxString CreateUniqueName(const wxString &prefix,
                              const wxString &suffix = wxEmptyString);
 
-   // Create a filename for an unsaved/temporary project file
-   wxString UnsavedProjectFileName();
-
    // File extension used for unsaved/temporary project files
    wxString UnsavedProjectExtension();
 
    AUDACITY_DLL_API
    bool IsOnFATFileSystem(const FilePath &path);
-   bool FATFilesystemDenied(const FilePath &path,
-                            const TranslatableString &msg,
-                            wxWindow *window = nullptr);
 
    AUDACITY_DLL_API
    //! Give enough of the path to identify the device.  (On Windows, drive letter plus ':')