Load icons on-demand

Load icons on-demand

Because loading icons takes a noticable performance, this code loads the
pixbuf on demand and only loads icons for rows that are visible. There
is a few caveats to this:
- The pixbuf cell renderer must report the proer size even if the icon
  is not yet loaded. This is achieved by setting a fixed size.
- On theme changes the cahced pixbufs and the cell renderer must be
  updated to conform to the new theme.