Improved robot display

Especially when holding something. Thing held is now centred over the
robot.

Also fixes a bug where something the robot changed size.
Improved robot animation

Heads towards centre of widget
Better error handling

For when taking something from a robot. But perhaps better to not
support it all.
Fixed problem with robots dropping on empty holes

The dropped item wasn't being removed from its container.

Also if a backside widget's JSON doesn't specify its location then 0,0
is used since otherwise can be off page.