[evolution-patches] 44258: only show non-disabled columns

[Mike Kestner <mkestner ximian com>]

The gal-view-etable uses e-table-config to select fields for the view.
It was allowing fields marked disabled in the model to be added to the
view, causing general mayhem with the tasks model.  The
e-table-field-chooser-item correctly ignored these columns, which meant
the "Add a column" dialog was working correctly.  This patch brings the
two into synch.

It also contains a couple of one liners to potentially clean up some
warnings lewing was seeing and to guard against potential crashes in
e-cell-text if the model returns NULL instead of an empty string. 

Mike