This adds back the implementation for setting the SymbolBitmapURL
property (getting returns an empty string) and adds SymbolBitmap
property to the UNO API (LineDiagram.idl).

SymbolBitmapURL property was already deprecated, but we still need
to add the SymbolBitmap for compatibility.

[ Miklos Vajna: updated qadevOOo tests to handle the new write-only
properties. ]

Change-Id: Id00d78299b7fd5ba9d78c36c5f03a10d9e61c60a
Reviewed-on: https://gerrit.libreoffice.org/53492Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>