Replace DOMPDF with TCPDF for "Create PDF Letter"

The action "Create PDF Letter" currently uses the library DOMPDF. The action "Create Name Badges" uses a newer library called TCPDF. "Create PDF Letter" needs to be updated to also use TCPDF. This will have the benefit of improving the code consistency, and allow for the use of "stationary" ie letterhead that needs to be used as the background for every page in the PDF file.

NOTE: Stationery path field has been suppressed in Contact/Form/Task/PDFLetterCommon.php since it's not functional with DOMPDF. That field should be reinstated and some help added when this issue is resolved. At that point the Stationery path value included in the PDF Page Format settings (currently that field isn't available under Administer > Communications > Print Page (PDF) Formats > Add PDF Page Format) so admin can set it for a given format and it get's populated for Print PDF Letter action if user selects a format that has it set.