Mass Edit issues #3 and #4

This PR addresses two issues with Mass Editing of attributes with the custom array element...  First (#3) was that the form elements weren't serialised correctly when saving.  The second (#4) was that there was no "Change" checkbox, so the user had no option but to change the value of any attribute using the array frontend.  The second issue is resolved by disabling the fields (so the browser won't submit them) when "change" checkbox is unchecked.  This mimics the method used by the "simple" controls in the core.
@aedmonds Good catch, yes I have.  I've committed that to master now.