Don't require that types be specified on the command line if the displayer

Don't require that types be specified on the command line if the displayer
doesn't require them.

For example, it's sensible for a GUI program to load without having any
assembly loaded, so there's no reason for assemblies/types to be specified.

This allows us to do:

    type-reflector --displayer=gtk

And get an empty Gtk# window (which can be later filled by using
File->Open...).

svn path=/trunk/mcs/; revision=10335