Truncate field labels in "Manage Dependencies" admin tab

Issue Summary

In a project I am currently working on, my fields have really long labels (it's a questionnaire) which makes the "Manage Dependencies" tab really wide and somewhat unwieldy. I have attached a quick patch to show what I did to mitigate this issue in case this is something that might be useful to someone else. I also removed the check_plain() from the labels that populate the select lists since it was being double-encoded (i.e. I don't think the check_plain() is necessary there).