Use better icons for data streams.

Change-Id: I57064ff251a560b71aaf1846a9d7cd9105f938df
Rework data streams to be more like file links (ScAreaLink).

By inheriting from sfx2::SvBaseLink and storing in sfx2::LinkManager we can
have more data streams and see / remove them in Edit -> Links... dialog.
Also rename to DataStream to avoid confusion.

Change-Id: I9c3b89020324af7be082f9e6e1cd479aeb72fe81
Import data streams - reuse infrastructure for ScAreaLink.

Change-Id: I18db991d2e8e665d5e4ca778344993e79772ccd5
export data streams; pretend to be an area link

Fix ScXMLExport::GetAreaLinks to not use UNO, so we can hack DataStreams
into ScMyAreaLinksContainer.
We need to connect them to cells for export / import - that does not make
sense for DataStream without any attached range. We use an arbitrary one.
But you might remove it, if you create another ScAreaLink / DataStream
with the same range, without knowing about it.

Maybe it wouldn't be that bad to let DataStream inherit from ScAreaLink.

Change-Id: I5b85a9329ba1ca46fb2893b54fe5161d2fb22f47
datastreams: do not hang when closing the link

Change-Id: Ib38472849d7b56bf3013f65d84f1b4cfbd91b2d0
Allow to edit data stream from Edit-> Links... dialog.

Change-Id: Ibb456da55f04c39ddb7b58eb71ca488cf209edb5