Print Dialog: Add support for the Common Print Dialog Backends (CPDB)

[Caolán McNamara]

On the technical side the things to look at might be:
a) The existing PrinterCapType::ExternalDialog "capability" which might
mesh with the concept here, which is used (in some circumstance 
apparently) on macOS; git grep ExternalDialog vcl
b) The now removed (since migration from gtk2 to gtk3) experimental
integration to use the gtk print dialog. This was removed in commit
1ae450504cf57457f9702684b1517fda1dd3c481 but might be worth a look as
something similar.

FWIW back in that experimental gtk2 print dialog, one of the nuisances
(and whose equivalent problem can be seen in a current version
gnumeric) is that for spreadsheet printing the selection of sheets to
print and/or cells couldn't be placed in proximity with the built in
page ranges. In gnumeric, using the standard gtk dialog, they are
placed a little awkwardly in a third tab, while in the LibreOffice non-
standard dialog they are grouped into the "range and copies" category.