move INetContentTypes::scan() to INetMIME::scanContentType()

to avoid circular dependencies between svl and tools when using
INetContentType::scan functionality for future handling of data urls in
urlobj.cxx

Change-Id: Iad13286769e8906aebf8208e4f532151ff2f3d13
Signed-off-by: Stephan Bergmann <sbergman@redhat.com>