webview: Add some debug code to show an inspector

This makes it easier to find the various nodes, and check why the
mobile view might not work.

Note that you'll need to remove the modality on gnome-control-center's
"add accounts" dialogue for this to work.

FIXME: Mouse and keyboard events don't seem to get to the inspector.

https://bugzilla.gnome.org/show_bug.cgi?id=704564