We hard-code a 16x16 asset for sunrise and sunset icons, which makes
them look blurry at higher scaling factors.
Let's look in the icon theme to see if there's an icon for sunrise and
sunset; if not, we can still use the assets we ship, but we also attempt
to load them from different resource paths, depending on the scale
factor.
We will need to update the assets to include a 32x32 rendering of the
sunrise and sunset icons.
https://bugzilla.gnome.org/show_bug.cgi?id=781029
