bindConditionalShow: fix for shortened fieldnames

If you had not shortened form name and shortened field name, then bindConditionalShow didn't work because it used form ID to generate field ID.
Some time ago data-shortname attribute for fields was introduced and now bindConditionalShow also uses this attribute to correctly find respective fields.
Merge pull request #309 from DarkSide666/patch-6

bindConditionalShow: fix for shortened fieldnames