Make FileLayout_XXX elements of an enum

Slight improvement to the previous commit: make FileLayout_Classic and
FileLayout_XDG elements of an enum instead of using an untyped "int" for them
which didn't really make any sense because these values are not bit masks but
exclusive choices for the layout.

Also rewrite the checks for them to use "switch" instead of "if" to be warned
by the compiler if we ever add another enum value but forget to update the
code to handle it.

Finally, improve the documentation (add missing "@since") and comments.

interface/wx/stdpaths.h

@@ -135,16 +135,34 @@ public:
         Dir_Videos
     };
 
-    enum
+    /**
+        Possible values for SetFileLayout() argument.
+
+        The elements of this enum correspond to the different file layout
+        standards under Unix systems.
+
+        @since 3.1.1
+     */
+    enum FileLayout
     {
-	/**
-            Use the classic file layout
-	*/
+        /**
+            Use the classic file layout.
+
+            User configuration and data files are located directly in the home
+            directory.
+
+            This is the default behaviour for compatibility reasons.
+        */
         FileLayout_Classic,
 
-	/**
-            Use a XDG styled file layout (Unix)
-	*/
+        /**
+            Use a XDG styled file layout.
+
+            File layout follows the XDG Base Directory Specification (see
+            https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html).
+
+            This is the recommended layout for new applications.
+        */
         FileLayout_XDG
     };
 
@@ -313,8 +331,11 @@ public:
     virtual wxString GetTempDir() const;
 
     /**
-        Return the directory for the user config files:
-        - Unix: @c ~ (the home directory)
+        Return the directory for the user config files.
+
+        This directory is:
+        - Unix: @c ~ (the home directory) or @c XDG_CONFIG_HOME depending on
+            GetFileLayout() return value
         - Windows: @c "C:\Users\username\AppData\Roaming" or
                    @c "C:\Documents and Settings\username\Application Data"
         - Mac: @c ~/Library/Preferences
@@ -339,7 +360,8 @@ public:
 
         If the value could not be determined the users home directory is returned.
 
-        @note On Unix this supports the xdg user dirs specification.
+        @note On Unix this method respects the XDG base directory specification
+        only if SetFileLayout() with @c FileLayout_XDG had been called.
 
         @since 3.1.0
     */
@@ -458,17 +480,24 @@ public:
     void UseAppInfo(int info);
 
     /**
-        Returns the current file layout
-	Valid values for @a are:
-         - @c FileLayout_Classic,
-         - @c FileLayout_XDG
+        Returns the current file layout.
+
+        The default layout is @c FileLayout_Classic for compatibility, however
+        newer applications are encouraged to set it to @c FileLayout_XDG on
+        program startup.
+
+        @since 3.1.1
     */
-    void SetFileLayout(int layout);
+    void SetFileLayout(FileLayout layout);
 
     /**
-        Returns the current file layout
+        Returns the current file layout.
+
+        @see SetFileLayout()
+
+        @since 3.1.1
     */
-    int GetFileLayout() const;
+    FileLayout GetFileLayout() const;
 
     /**
         Return the file name which would be used by wxFileConfig as local,