Fix calculation of wxStyledTextCtrl selection size in GetSelectedText().

Do the same changes as were done in r71540 to GetSelectedTextRaw() in
GetSelectedText() itself by modifying the code in gen_iface.py that generates
it.

Closes #14331.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71556 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/stc/stc.cpp

@@ -1504,11 +1504,7 @@ bool wxStyledTextCtrl::GetModify() const
 
 // Retrieve the selected text.
 wxString wxStyledTextCtrl::GetSelectedText() {
-         long   start;
-         long   end;
-
-         GetSelection(&start, &end);
-         int   len  = end - start;
+         const int len = SendMsg(SCI_GETSELTEXT, 0, (sptr_t)0);
          if (!len) return wxEmptyString;
 
          wxMemoryBuffer mbuf(len+2);