Only focus subscription URL for "Add".

Hi again Bob,

Here's a small thing that's been bugging me.  Perhaps it bugs you and/or others too.

Whenever you edit a subscription, the URL text box gets the focus.  So the keyboard jumps onto the screen.  Usually, that's not what I want.

You can get to this activity in one of three ways:

- From the subscription list (`Edit`); in which case we probably don't want to edit the URL, rather change one of the other settings.  So don't focus the URL.

- From the `Add` entry on the subscription list menu; in which case it *does* make sense to focus the URL; that's probably where the user will type/paste next.

- From externally; in which case we already have a URL, so we're unlikely to want to change it; so don't focus it.

I promise.  After this, I have just one change I'm interested in proposing.  Well, for the time being.  So I'll be off your back soon.

Thanks for the great work!

Steve