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.