bug#19226: eww.el desktop support fixes: autoload eww-mode, use inhibit-read-only

[Glenn Morris]

Come to think of it, desktop mode could solve this itself:

When restoring foo-mode, if foo-mode is not bound, try requiring foo.
That would solve this case and many others.

For a complete solution, when desktop saves a major/minor mode to
restore, it could also save the name of the library that defines the
mode. When restoring, first of all require that library.