storage-view: Port to Gtk+ composite templates

It simplifies the StorageView code a lot by decoupling the Gtk+
specific bits.

In terms of logic, we now switch stack pages to present the
"loading", "content", and "empty" states. This is simpler than
the addition and removal of widgets on callbacks.

https://bugzilla.gnome.org/show_bug.cgi?id=787357