Better resize of expandable columns

Better resize of expandable columns

Since 16195ad the “expand” property is always set to FALSE when a
column is resized.  This commit takes a different approach and enables
“expand” whenever the column is wide enough.  An appropriate
“fixed-width” (so that the desired width is achieved after expanding) is
calculated using equations that are explained in the code.

https://bugzilla.gnome.org/show_bug.cgi?id=691751