Need public interface to download management

[Kang Jeong-Hee <Keizi mail co kr>]

> Epiphany has a download manager built into it.

And there's no public way to get notified about downloadings.

EphyEmbedShell provide a pointer to downloader view, but the GObject
does not emit any signal such as download added, removed, cancelled,
or done.

Particular download job does not appears in public.

I'm writing an extension which is port of downloadstatusbar.mozdev.org
And have put an "added" signal into downloader view to get notified.

Such a public interface to download management is really necessary.

kz.