Fix pasting of PDF from clipboard

Fix pasting of PDF from clipboard

The command "paste pdf" was always disabled because the
condition in the following "if" statement always returns false

  if (arg == "pdf" && (type = Clipboard::PdfGraphicsType))

The value of "type" is zero in this case because PdfGraphicsType is
the first enum value (and it is not set explicitly to non-zero).

An alternative patch is to put AnyGraphicsType as the first
element of enum GraphicsType, or to set the first element to a
number greater than 0.

To test the bug that this commit fixes, either copy a PDF and try to
paste with the action "paste pdf", or click on the "Edit" menu and
notice (before this commit) the terminal output "Unrecognized
graphics type: pdf".