Add wxStandardPaths::GetUserDir() to get Desktop, Download etc

All major supported platforms have well defined per-user directories to store
Downloads, Music, Pictures, Videos and the Desktop files. The new method
wxStandardPaths::GetUserDir() allows for a unified way to access these on MSW,
OS X and Unix (if XDG user dirs specification is implemented for the latter).

See https://github.com/wxWidgets/wxWidgets/pull/89

src/common/stdpbase.cpp

@@ -114,11 +114,6 @@ wxString wxStandardPathsBase::GetUserLocalDataDir() const
     return GetUserDataDir();
 }
 
-wxString wxStandardPathsBase::GetDocumentsDir() const
-{
-    return wxFileName::GetHomeDir();
-}
-
 wxString wxStandardPathsBase::GetAppDocumentsDir() const
 {
     const wxString docsDir = GetDocumentsDir();
@@ -133,6 +128,11 @@ wxString wxStandardPathsBase::GetTempDir() const
     return wxFileName::GetTempDir();
 }
 
+wxString wxStandardPathsBase::GetUserDir(Dir WXUNUSED(userDir)) const
+{
+    return wxFileName::GetHomeDir();
+}
+
 /* static */
 wxString
 wxStandardPathsBase::AppendPathComponent(const wxString& dir,