jens: unit filters

[GoogleCodeExporter]

allow columns to be filtered - but for units to be filtered it would be nice to 
filter LOCATION by nearness to a certain point - and also to filter unit type 
(which is currently a group header rather than a column).

It is VERY common for me to want to find my nearest sub to a given  enemy 
battleship, or my nearest destroyer with enough movement points to a given 
enemy sub

Original issue reported on code.google.com by [email protected] on 20 Jun 2011 at 2:28