svx_unoapi svx.AccessibleShape on Windows:
One thread in ChildrenManagerImpl::GetChild() calling eventually
SdXShape::getPropertyValue() blocking on SolarMutex,
other thread calling ChildrenManagerImpl::CreateListOfVisibleShapes().

Not obvious if it's possible to release the maMutex in GetChild() around
calling ShapeTypeHandler::Instance().CreateAccessibleObject(),
since other methods here call GetChild() with maMutex locked themselves;
so just give up and use the SolarMutex in ChildrenManagerImpl too.

Change-Id: I9bf4e5926c4d01308513b1b4e88f20f9fd4c5648