Use SHA1 data hash in icon cache paths (Fixes #31)

This patch fixes the issue of many hundreds or thousands of duplicate small files piling up over time in the notify log's icon cache. A common example case where this issue occurs is to have Gmail desktop notifications enabled in a web browser.

GLib-2.64 SHA-1 is faster than both of its MD5 and SHA-256 implementations. If Glib's SHA-256 becomes as fast as SHA-1 then the source can be updated to use SHA-256 instead of SHA-1, as the software industry is slowly moving away from SHA-1 towards using SHA-256.