Add wxT_2() macro for compatibility with wxWidgets 2 API.

This macro can be used to make the same code compile with both v2 and v3 as it
expands to wxT() in 2.8 and nothing in later versions.

See #12925.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66968 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/chartype.h

@@ -32,6 +32,32 @@
 */
 #define wxT(string)
 
+/**
+    Compatibility macro which expands to wxT() in wxWidgets 2 only.
+
+    This macro can be used in the code which needs to compile with both
+    wxWidgets 2 and 3 versions in places where v2 API requires a Unicode string
+    (in Unicode build) and v3 API only accepts a standard standard narrow
+    string as in e.g. wxCmdLineEntryDesc structure objects initializers.
+
+    Example of use:
+    @code
+    const wxCmdLineEntryDesc cmdLineDesc[] =
+    {
+        { wxCMD_LINE_SWITCH, wxT_2("q"), wxT_2("quiet"),
+          wxT_2("Don't output verbose messages") },
+        wxCMD_LINE_DESC_END
+    };
+    @endcode
+
+    Without @c wxT_2 the code above wouldn't compile with wxWidgets 2, with @c
+    wxT instead of it, it wouldn't compile with wxWidgets 3.
+
+    @see wxT()
+
+    @header{wx/chartype.h}
+ */
+
 /**
     wxS is macro which can be used with character and string literals (in other words,
     @c 'x' or @c "foo") to either convert them to wide characters or wide strings