Add a helper to handle captive portal logins

Add a small DBus-activated GtkApplication that embeds a WebKitWebView
and implements some minimal logic to see if the login succeeds.
It will try to connect to a custom NM-provided url (the portal login
page), if one exists, or to www.gnome.org in the normal case of
a portal doing redirect.

https://bugzilla.gnome.org/show_bug.cgi?id=704416
NetworkManager: show portal logins when required

Listen to changes in connectivity, and ask our helper to authenticate
when needed.
We don't have a URL to connect to yet (we will have when
the new NM API lands), so we use the default of trying
www.gnome.org (which is also more reliable because we can
recognize when the login is done)

https://bugzilla.gnome.org/show_bug.cgi?id=704416