Add Subject and Campaign fields to Print PDF Letter activity form

See forum post: http://forum.civicrm.org/index.php/topic,27349.0.html

Request is to allow users to add an activity subject value when they choose the "Print PDF Letter" activity option, so this value would appear in the subject field of the list of activities in the Activities tab, and could also be used in Advanced Search.

Request is also to allow users to associate a "Print PDF Letter" activity with a campaign.

Both fields to be optional, to support backward compatibility.

Proposed solution involves editing the following files to add the fields to the Print PDF Letter form, and have them included in updates to the activity record:
CRM/Contact/Form/Task/PDFLetterCommon.php
templates/CRM/Contact/Form/Task/PDFLetterCommon.tpl