SwDoc::dumpAsXml: show mbModified

Change-Id: I491d4af104453bb5ff3b55b28e3e1775d8aab8f7
SwAnchoredDrawObject::GetObjBoundRect: avoid SwDoc::SetModified()

This is a const method, but it does a const_cast to still resize an
object... if that's so, then we should ensure that the "is modified"
flag of SwDoc is untouched.

CppunitTest_sw_ooxmlimport's testChartSize is a reproducer for this,
when shape text is imported as textbox.

Change-Id: I6dedb31520a14127ca91782874c285f6e0f93c65