Replace the cell data computations

Replace the cell data computations

Previously custom functions were used to compute the data passed to the
cell renderers. Now that all this data is saved by the tree models with
compatible nodes, the usual default attribute-to-column mapping can be
used.
With this, caching of the values can happen in the tree model, which
avoids costly lookups of icons or computation of strings. Last but not
least it avoids spurious bugs that could happen when strings changed
without anyone noticing, like the mtime when a new day begins.