tdf#93437: Re-enable input after closing formula wizard

Input was disabled on opening the wizard (in ScFormulaDlg ctor),
but never enabled again. Row and column headers seem to be
more sensitive about it than e.g. grid window and they then
just discard mouse events ...

Change-Id: Id27757fb1b3a8b96ad1588354f016ac55d71cca2
Reviewed-on: https://gerrit.libreoffice.org/17872Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>

sc/source/ui/formdlg/formula.cxx

@@ -255,6 +255,7 @@ void ScFormulaDlg::dispose()
         pScMod->SetRefInputHdl(NULL);
         StoreFormEditData(pData);
     }
+    ScFormulaReferenceHelper::enableInput( true );
     formula::FormulaDlg::dispose();
 }