Fix removal of rectangles and electric fields, dispatch new events

[#48435497]
Add automated tests of electric fields

[#48435497] [#52998997]

Also test their relation with rectangles, especially when rectangle
is removed.
Dispatch 'changeElectricField' event when electric field is updated

[#52998997]

+ update automated tests.
Let users specify color of the electric field visualization

[#53553247]
Update "Electric Field" interactive

[#53553247]
support interactives with no model at all

example:
/embeddable.html#interactives/basic-examples/no-model.json

Doesn't work yet with interactives.html.
support Interactives w/o model, improve not-found error
Add example of model defined in Interactive