Feature Request: Adding Subpile - Allow us to filter by group

[sarahd1987]

When selecting stockpiles to add as a subpile, it'd be great to be able to filter them by which group they're in.

[GoldenGnu]

Shouldn't be too hard and makes a lot of sense

Ref:
https://explodingpixels.wordpress.com/2009/08/05/creating-a-separated-jlist-using-glazed-lists/