Improvements for visualisation

[stonier]

The display module has a few problems.

Must-solve

• The updated status of each and every ticked behaviour must be reflected for visualisation purposes and not lost. This currently occurs when composites / decorators invalidate behaviours after the behaviour tick but before visualisation for logistical reasons to avoid dangling runners. Refer to Loss of visibility when invalidating #132 for an example.

Nice-to-haves

• Differentiate between not-ticked and ticked parts of the tree (e.g. gray unticked, white ticked, cyan tip). Only relevant for unicode tree display methods (py_trees_js already does this).
• A toggle to show the last non-invalid status for behaviours, even beyond the last tick.

Potential Solution

Do logistical cleanup just prior to ticking the tree (might be expensive / awkward) OR cache the last non-invalid status and use that as the status to visualise for ticked behaviours.