[Libreoffice] [REVIEW][3-5] sequence of unfortunate writer ole object in headers problems

So https://bugs.freedesktop.org/show_bug.cgi?id=43867 is a most annoying
bug with a crash on mailmerging documents that contain ole objects in
headers. I'd like to get the following into 3-5.

Immediate crash is fixed by
http://cgit.freedesktop.org/libreoffice/core/commit/?id=7dc25878c1310a1de6d3c87c88404b151bad21fc
where we previously acquired/released the document around the insertion
of content into it, but not around inserting styles, but page styles can
contain headers/footers with arbitrary content, so if we need to acquire
before inserting content, we need to acquire before inserting page
styles.

object preview is still missing from headers though,
http://cgit.freedesktop.org/libreoffice/core/commit/?id=ef17be8b006737c078a59635ae334a03301727ea
fixes a series of apparent errors

1. FN_UNO_REPLACEMENT_GRAPHIC_URL and FN_UNO_GRAPHIC have the same id in
writer, so "replacement graphic urls" fall into the same case statement
as "replacement graphics", which they aren't. These random ids are *so*
fragile, we need something better here in general.
2. The replacement graphic is set with...
svt::EmbeddedObjectRef xObj = pOleNode->GetOLEObj().GetObject();
xObj.SetGraphic( aGraphic, aMediaType );
but the x of xObj and the name "EmbeddedObjectRef" are misleading, its
not a reference class, so we're setting the replacement graphic onto a
temporary copy of the object. EmbeddedObjectRef is a poor class name, we
should change that.
3. SvXMLGraphicHelper::ImplGetStreamNames doesn't know how to
split ./ObjectReplacements/foo in order to find the preview but
SvXMLEmbeddedObjectHelper::ImplGetStorageNames does, so factored out
better splitter and shared it 

C.