CRM-13296 fix e-notice, change cancel button text on mailing labels to D...

...one, refactor to sare buildQuickForm common code. Note that more refactoring is probably needed on this (4.5?) to eliminate duplicated code in postProcess, setDefaults - making use of LabelCommon.php.

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* CRM-13296: Add tasks to membership search - print labels & print pdf letter
  http://issues.civicrm.org/jira/browse/CRM-13296