Improve show/hide behaviour in the Blueprint Tree UI

[abey79]

In the Blueprint Tree UI, it's currently impossible to unhide a single child. To achieve that effect, the parent must be unhidden and all the siblings hidden, which is extremely cumbersome. This should be done automatically when the user tries to unhide a children.

[h3mosphere]

This is indeed quite tricky to get correct, the UI should probably behave like so:

If a parent group is hidden by the user, keep the current behavior WRT to the blueprint view state. Therefore if the user unhides the parent, the children will keep the visibility state that may have been previously, manually chosen by the user.

If a parent group is hidden, and then a child entity/group is then unhidden, then the code should walk to the most distant hidden parent group, and hide all the child groups & entities, except for the ancestor chain from the newly unhidden group/entity. Either by keeping a Vec of direct ancestors, and not hiding those ones, or hiding all children, and then walking again up the ancestor chain, unhiding until reaching the most distant hidden parent group. Hope this makes sense.

A tree walking visitor pattern would probably be nicest to use for this, with bailout return values.

[abey79]

Recent discussions with @jleibs on next steps around blueprint indicate that a significant overhaul of the blueprint tree (and its UI) is going to happen sooner than later, so it's best to hold off any work in that area for now.