background: Add a placeholder icon before creating thumbnails

We do not wait until the first thumbnail is created to start filling up
the store. This avoids the "No Pictures Found" message from showing up
on slow machines were I/O and decoding the data may take a while.

However, placeholders are not shown for PNGs since they might be
screenshots.

https://bugzilla.gnome.org/show_bug.cgi?id=708943