Make the position of the filter icon customizable with respect to the existing icon for the column label.

Currently, when a filter icon appears it forces the existing icon to appear on the leading edge of the text. This is not desired in cases where one would prefer that the icon stays on the trailing edge of the text (e.g. the behavior of the JXTable header renderer). This change makes it so that the filter icon's placement relative to the existing icon is customizable, but it does not cause the existing icon to switch to the other side of the label.