Improve the favicon cache. It has a variety of problems at the moment:

In theory we only need to store images on disk for favicons that are needed not as a result of a page load. This means that just bookmarks (for the moment, history doesn't use them at the moment) will need them stored on disk.

What I propose is:

When a page is loaded that has a favicon:

When an object wants a favicon

Peridocally, the favicon cache will cull the GdkPixbuf?'s from memory that haven't been used in a while, it will also save images that are used for bookmarks to disk.

To download icons, we should use something like tko's code in although we need to check to see whether that uses the mozilla cache.