wizard: Unselect entry when going from URL to main page

If you follow these steps, 'Continue' becomes sensitive:

1. Type something in the entry on URL page
2. Go back to main page
3. Go to URL page
4. Remove all text from entry
5. Repeat 2 and 3 again

This patch fixes the problem by simply unsetting the current selection
when going back from URL to main page. IMHO this makes sense anyways
since on main page the URL is not shown so user doesn't know what would
'Continue' imply. Note that with this patch, 'Continue' becomes
sensitive again if you go to URL page and there is text entered already
in there.

NOTE: We probably should be doing some more checks on URLs before
enabling the 'Continue' button rather than simply checking non-zero
length string.

https://bugzilla.gnome.org/show_bug.cgi?id=684220