Use cell renderer to show thumbnail loading icon

Instead of storing the image-loading icon in the model, handle it in the
view by setting the icon name property of a pixbuf cell renderer.
Handle failed ImageWorker tasks

When a thumbnail task fails, display a generic image icon.
Show error message for failed image loads

Use a GtkStack to switch between the image view and a message area.
Remove 'failed' from Task, check pointer instead

A separate 'failed' bool isn't necessary - check if the RefPtr is empty
instead.
Async folder opening in ImageList

Convert ImageList to open folders asynchronously. The implementation is
a bit more complex, but makes the UI more responsive for big folders.
Impovements to folder opening and messages

* Add messages for folders with no images and folders that could not be
  opened.
* Select the first image after opening a new folder.
* Fix some cases of stale info in the UI, like a header subtitle or
  message belonging to an image from a previous folder.
Handle command line options

Handle the standard GTK+ command line options, and --version.