Bug 8064: Change the way target record is built.

Bug 8064: Change the way target record is built.

Instead of copying/removing a piece of DOM in target record each time a
checkbox is checked/unchecked, the target record is *entirely* rebuilt
each time a checkbox is checked/unchecked.
This is slower but allow for a more consistent and less error-prone
behaviour.

This patch also fix the mandatory check for subfields

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>