Implement KatzeNet as a generic URI loader

Implement KatzeNet as a generic URI loader

KatzeNet can from now on be used where
URIs are handled and one would have used
libSoup and/ or local file handling as
appropriate, whereas KatzeNet does both.

Since we are displaying icons in several
places KatzeNet also provides an icon
loader that saves us from doublicating
even more code.

All bookmarks and also history items have
icons now, since KatzeNet makes that
incredibly easy.

Search engines are also using favicons
now, and right now custom icons don't
work, we still need to fix that.

Note that only icons are cached, nothing
else and the code is known to have a
hidden, hard to reproduce crasher which
may appear when an icon is newly loaded.