Draw relationship lines for only selected objects

In scenes with lots of objects and relationships, relationship lines can get quite messy. It's possible to turn them off, but complex scenes are exactly where seeing relationships can be the most useful. If we were to only see the lines for selected objects, it could be the best of both worlds. We already do this for bones in pose mode, so it's not much of a stretch.

It might also be helpful to draw the lines for all child objects in the hierarchy of the selected object.

In the example below, if you have Bone 1 parented to Bone 2 and Bone 2 parented to Bone 3, moving Bone 3 will move all three bones but only the connection between 3 and 2 is shown. It may be useful to see the lines that connect everything that would be affected by a transformation.