Fix exit crash when showing GDB options page.

The options page deleted the widget, which it does not own.

Remove the deletion, rename GdbOptionsPagePrivate to
GdbOptionsPageWidget and use a QPointer in the GdbOptionsPage,
rename the member to m_widget for clarity.

Change-Id: I211a7cf4d59015c540335bd6313eaff13f1999a4
Reviewed-by: hjk <qthjk@ovi.com>