search: add support for default disabled search providers

Search providers that should be disabled by default come with
a DefaultDisabled=true key in their keyfile, and are enabled
with the "enabled" whitelist, not with the "disabled" blacklist.

https://bugzilla.gnome.org/show_bug.cgi?id=734110
SearchEntry: replace the ID with a style class

This way extension authors can create entries that look and feel
like the overview search entry.

https://bugzilla.gnome.org/show_bug.cgi?id=733813
PortalHelper: fix portals that don't redirect properly

We assumed that either a portal would give us a target URI, or we
would eventually reach the URI of our choice (which is http://www.gnome.org)

This is not always the case: even after a successful login we may
stay in a confirmation page, so make sure that every time we see a
redirect we trigger a connectivity check (or queue one for when
the user closes the window)

https://bugzilla.gnome.org/show_bug.cgi?id=733848