item-manager: Handle failures better

Emit "active-collection-changed" and update GtkRecentManager only when
the parent's invocation of set_active_item has succeeded.

Fixes: https://bugzilla.gnome.org/737071
item-manager: Add "load-started" and "load-finished" signals

This keeps the code in sync with gnome-documents.
preview-view: Disable the sliding transition

Sometimes we hit a slow path in GEGL when rendering which makes the
transition jittery and look broken. Let's disable it till we have a
solution to make it smoother.

Fixes: https://bugzilla.gnome.org/726505
embed, spinner-box: Overlay the spinner

... instead of using a separate view with an opaque background.

Fixes: https://bugzilla.gnome.org/737425
embed: Switch to the preview as soon as an item starts loading

We are essentially in the preview, not the overview, when an item
starts loading. Therefore the UI should relate to the current context.

Fixes: https://bugzilla.gnome.org/734845
Let the back button cancel ongoing loads

Original patch from Pranav Kant for gnome-documents.

Fixes: https://bugzilla.gnome.org/734845
Don't show a glimpse of the older item when going back to the preview

Now that our spinner is overlaid and we switch to the preview as soon
as it starts loading, we should take care to clear any older item that
might have been previewed earlier.

Fixes: https://bugzilla.gnome.org/734845
preview-nav-buttons: Skip invalid paths

Now that we can quickly navigate through a bunch of slow loading items
in the preview (due to the spinner being overlaid), we can hit an
invalid path towards the beginning or end of the model.

Fixes: https://bugzilla.gnome.org/737425