In order to wrap the SdrPage's UNO object completely, set its mxUnoPage
member to the OSection wrapper instance in OSection::init; only OSection
should have access to it.
Also initialize m_xDrawPage_Tunnel (thanks Lionel for the hint).
(regression from 05218c10)

Change-Id: I048ddafc31e946853e56e6a403ddc9487cfbcf0e