Add ability to ask for numerical value on drop.

If the input value (with optional margin) is set for a drop item,
an input field is shown and focused when the user drops the item
on the correct zone.

The value is submitted on field blur. If correct (within the margin),
the field is colored green. If the value is not correct, the field is
colored red. In either case, the field gets disabled and user cannot
try again until resetting the exercise.

The exercise is not considered finished until all required input
fields are filled.

If the item has an input set, the user only gets points for the item if
the input value is within the correct margin.
I know that on some functions, we don't really have that much to add in the way of docstrings, but I'd prefer we stick with one way of writing them. PEP8 recommends a triple quoted string, a line break, the text, and then the other triple quoted string on the next line. This makes adding more to the docstring easier later.

This is a nitpick, but it's better that we just pick one style of docstring and stick with it. We have two or three here.