Define forward-compatible wxT_2() macro.

This macro is the same as wxT() in wx 2.8 but allows to write code compatible
with wxWidgets 3 where it expands to nothing.

See #12925 and r66968.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@66970 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -96,6 +96,9 @@ All:
 - Fixed a bug in UNC path testing.
 - Improved command line parser usage help.
 - Fixed race condition bugs in wxCondition.
+- Added wxT_2() for forward compatibility with wxWidgets 3. Use it in
+  initialization of wxCmdLineEntryDesc struct elements and (very few) other
+  places where wxT() is required currently but won't be allowed in v3.
 
 All (GUI):
 

include/wx/wxchar.h

@@ -237,6 +237,14 @@
     #define wxT(x) x
 #endif /* Unicode/!Unicode */
 
+/*
+    This macro is defined for forward compatibility with wxWidgets 3. It should
+    be used in the places where wxWidgets 2 API requires wxT() (in Unicode
+    build) but wxWidgets 3 doesn't accept it, e.g. wxCmdLineEntryDesc struct
+    elements initializers.
+ */
+#define wxT_2(x) wxT(x)
+
 /*
     We define _T() as a synonym of wxT() for backwards compatibility and also
     for the benefit of Windows programmers used to it. But this identifier is a