Drop class utl::MediaDescriptor, and use comphelper::SequenceAsHashMap

utl::MediaDescriptor was just a SequenceAsHashMap with a few added
methods. It made it more difficult to assign a SequenceAsHashMap to
MediaDescriptor, without real upsides. In many places, the class was
used not as media descriptor, but just as helper to create sequences.

This change puts the string constants and the former methods into a
namespace utl::MediaDescriptor.

It would be also good to drop comphelper::NamedValueCollection, which
is basically a twin of comphelper::SequenceAsHashMap (TODO).

Change-Id: Ia9b69c5a2ad153ecc654944c419c1880252ed86e
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/195163
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

xmlsecurity/source/helper/pdfsignaturehelper.cxx

@@ -26,7 +26,7 @@
 #include <comphelper/propertysequence.hxx>
 #include <sal/log.hxx>
 #include <comphelper/diagnose_ex.hxx>
-#include <unotools/mediadescriptor.hxx>
+#include <comphelper/sequenceashashmap.hxx>
 #include <unotools/streamwrap.hxx>
 #include <unotools/ucbstreamhelper.hxx>
 #include <vcl/filter/pdfdocument.hxx>
@@ -92,7 +92,7 @@ void GetSignatureLineShape(const uno::Reference<frame::XModel>& xModel, sal_Int3
     }
 
     // Export just the signature line.
-    utl::MediaDescriptor aMediaDescriptor;
+    comphelper::SequenceAsHashMap aMediaDescriptor;
     aMediaDescriptor[u"FilterName"_ustr] <<= u"draw_pdf_Export"_ustr;
     SvMemoryStream aStream;
     uno::Reference<io::XOutputStream> xStream(new utl::OStreamWrapper(aStream));