Simplified and optimised removal from nest

shift is clearer and faster than splice(0, 1)
Display of erased boxes fixed

And improved display of box holes. Removed commented out CSS code.
Simplified vacuum tool

Since erase mode toggles erasure no need for restore mode to restore
erasures.
Implemented new display of vertical boxes
Fixed bugs in a bird leaving a box
Widget dimensions restored

When taken out of a box or a nest.
Birds when drag over animate give me

Previously this only happened after the bird has done something or has
been moved.
More controlled removal of robot children

Instead of removing all just removes widgets. Better, for example, if
robots were resizable.
Widget sensors now added to backside

When created.
Fixed typo.
Make a sensor button improved

Used to add the newly made sensor to the backside of the widget it is
sensing which led to circularities. And wasn't very useful. Also
abstracted the code for adding a widget to the top-level background.
Widgets restored by vacuum placed better

They are left of the vacuum rather than their original location.
Updated compiled version
Fixed problems training robots to give birds things

When bird is in a box it temporarily leaves the box and that broke its
training.
Robots did not remove widgets from backside

If they picked them up from there.
Robots didn't stop carrying the wand

Once the wand was picked up it kept displaying the wand.
Birds in robot conditions no longer shared

They were making a copy passing the nest causing inappropriate sharing.
Fixed a bug vacuuming off backside
Fixed a bug training a robot to drop on empty hole
Birds returning to a box update top container

Top-level widget still had bird as one if its backside widgets.