SmViewShell: pImpl must outlive aGraphic member

...as at least during CppunitTest_starmath_qa_cppunit, during ~SmViewShell,
aGraphic's code calls back into SmViewShell's code calls into pImpl:

 SmViewShell::IsInlineEditEnabled() const starmath/source/view.cxx:2057:12
 SmGraphicWindow::IsInlineEditEnabled() const starmath/source/view.cxx:227:12
 SmGraphicWindow::CaretBlinkStop() starmath/source/view.cxx:297:10
 SmGraphicWindow::~SmGraphicWindow() starmath/source/view.cxx:126:5
 SmViewShell::~SmViewShell() starmath/source/view.cxx:1976:1
 [...]

This is somewhat symmetric to 0e263d61811480f3a9dbc6ad7c752f6cfae4e3fc "order
pImpl before aGraphic because aGraphic deferences pImpl in ctor."

Change-Id: I97494bebaf5747205a690e6d65fd05ba8fb61cab