make a copy of Converters because this function is used by our export/preview threads

make a copy of Converters because this function is used by our export/preview threads
and the converters is also used by the main thread (getStatus).

LyX.exe!lyx::Buffer::doExport
LyX.exe!lyx::Buffer::preview(
LyX.exe!std::tr1::_Pmf_caller3<
LyX.exe!std::tr1::_Pmf_caller3<
LyX.exe!std::tr1::_Callable_pmf<
LyX.exe!std::tr1::_Bind3<
LyX.exe!std::tr1::_Bind_base<
LyX.exe!lyx::frontend::GuiView::GuiViewPrivate::runAndDestroy<
LyX.exe!lyx::frontend::GuiView::GuiViewPrivate::previewAndDestroy(
LyX.exe!QtConcurrent::StoredFunctorCall3<
LyX.exe!QtConcurrent::RunFunctionTask(
QtCored4.dll!QThreadPoolThread::run(
QtCored4.dll!QThreadPrivate::start(
msvcr100d.dll!_callthreadstartex()
msvcr100d.dll!_threadstartex(void * ptd)

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@37292 a592a061-630c-0410-9148-cb99ea01b6c8