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/registry.cpp

@@ -1057,7 +1057,7 @@ bool wxRegKey::SetValue(const wxString& szValue, const wxString& strValue)
       m_dwLastError = RegSetValueEx((HKEY) m_hKey,
                                     RegValueStr(szValue),
                                     (DWORD) RESERVED, REG_SZ,
-                                    (RegString)static_cast<const TCHAR *>(strValue.t_str()),
+                                    (RegString)wxMSW_CONV_LPCTSTR(strValue),
                                     (strValue.Len() + 1)*sizeof(wxChar));
       if ( m_dwLastError == ERROR_SUCCESS )
         return true;