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

js/js-resources.gresource.xml

@@ -26,6 +26,8 @@
 
     <file>perf/core.js</file>
 
+    <file>portalHelper/main.js</file>
+
     <file>ui/altTab.js</file>
     <file>ui/animation.js</file>
     <file>ui/appDisplay.js</file>