If fixed_width is not -1, sets the fixed width of this; otherwise unsets it.
The effective value of fixed_width is clamped between the minimum and maximum width of the column; however, the value
stored in the “fixed-width” property is not clamped. If the column sizing is gtk_tree_view_column_grow_only
or gtk_tree_view_column_autosize, setting a fixed width overrides the automatically calculated
width. Note that fixed_width is only a hint to GTK; the width actually allocated to the column may be greater or less than
requested.
Along with “expand”, the “fixed-width” property changes when the column is resized by the user.
| this |
A `GtkTreeViewColumn`. |
| fixed_width |
The new fixed width, in pixels, or -1. |