app portgroup: callback not used, causing issues when combined with other pgs

[mascguy (Christopher Nielsen)]

In cases where other portgroups are also used - such as python - this can result in issues. For example, it's currently necessary to place the inclusion for this pg at the tail end of port gramps, otherwise post-destroot handlers are run in the wrong order.

This is easy enough to fix. However, we need to ensure it doesn't cause unexpected issues in any ports. (Hopefully such cases are few and far between, but we should still try to identify ahead of time.)