Robots can now refer to backside widgets

Only works for first one found of each type. Fixes #40.
Better drop error handling

Drops now preventDefault immediately since any error may cause the
browser to go to a new page losing the work so far.
Run button label unchanged if disabled
Improved drop location of fresh elements

They don't have drag_offsets and the upper left corner ended up where
the mouse was at drop time.
Unhandled drops handled

By adding the dropee  to containing backside.
Can now drop robots in boxes

Actually any widget even those without a 'drop_on' method.
Can now drag text from the backside

Previous treated it as if the entire backside was being dragged.
Backside highlighted if frontside clicked

And backside already visible
parent split to front and back parents

Each widget now knows its frontside and backside parents and whether the
parents are front or back sides.
Better default size for top-level backside
Backside widgets now retain which side

Since either frontside or backside or both can be on the backside of a
widget.

Also can drag backside to another window. And its 'hide' button displays
the frontside if it isn't visible.
Backside background colour mirrors widget

Easier to distinguish and looks nicer.
Cross-window drag and drop of backsides

Including the top-level backside.
'Anima-gadgets' working

Robots running on the back a widget that is in turn on the back of a
widget now runs in the context of the outermost widget.
Change CSS transition curve

Better to ease in and out slowly but travel fast. And moving back was
cute but confusing.
Showing and hiding backside animates

Looks nicer and is less confusing.
Drop position when scrolled corrected

Using pageX instead of clientX, etc. ensures the drop ends up at the
right place. Fixes #49.

Also did some code cleanup.
Clicking on a resource runs it

And clicking elsewhere on a page without a top-level backside turns off
all widgets.
New version of a test file
JQuery UI now includes accordion

And menu is removed since not used.