Stockpile groups behave incorrectly with filters

[GoldenGnu]

1. Stockpile groups can incorrectly become hidden when collapsed with matching items in the group. This is due to the items being replaced by the IgnoreItem, that does not match anything.

Instead of using the IgnoreItem simply putting all the items in the same separator resolve the issue.

2. The stockpile group are not rendered if the first stockpile in the group is filtered out.

Recalculate the first group on table changes works, but, might be expensive? (It may be possible to minimize the amount of updates needed, given some effort)