Add wxMSW_CONV_LPCTSTR() and related macros and use them in wxBase.

Add macros hiding the ugly casts needed to pass wxStrings to Windows API
functions and use them in a couple of places in wxBase to simplify the code.

Closes #14338.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71636 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/dde.cpp

@@ -1068,7 +1068,7 @@ static HSZ DDEAtomFromString(const wxString& s)
 {
     wxASSERT_MSG( DDEIdInst, wxT("DDE not initialized") );
 
-    HSZ hsz = DdeCreateStringHandle(DDEIdInst, const_cast<wxChar*>(static_cast<const wxChar*>(s.t_str())), DDE_CP);
+    HSZ hsz = DdeCreateStringHandle(DDEIdInst, wxMSW_CONV_LPTSTR(s), DDE_CP);
     if ( !hsz )
     {
         DDELogError(_("Failed to create DDE string"));