Don't report loading progress; use a deadline and spinner pattern instead

Per #31, the status bar should not report the progress of loading operations, since the status bar itself may go away.

This is a meta-bug for moving from a "report loading progress all the time" model, to a model where EOG starts loading an image, and only displays a loading spinner if the loading operation takes longer than a short amount of time (say, more than one or two seconds).

As part of implementing that, I'd like to take a look at actually using the EogJob's cancellable during loading; it is not passed to the librsvg or gdk-pixbuf functions that do the actual work, and it is just checked in the read loop. (The read loop will go away, too, since the idea is to move to GdkPixbuf's streaming API.)

Tasks:

• #30 (closed) - use librsvg's stream API
• #43 - Use gdk-pixbuf's stream API
• #44 - Use the job's cancellable during image loading
• Implement a deadline and spinner