Don't rely on TARGETS to read selection targets for Motif DND

Don't rely on TARGETS to read selection targets for Motif DND

* lisp/x-dnd.el (x-dnd-types-alist):
(x-dnd-known-types): Fix formatting.
(x-dnd-xm-unpack-targets-table-header):
(x-dnd-xm-read-single-rec):
(x-dnd-xm-read-targets-table):
(x-dnd-xm-read-targets): New functions.
(x-dnd-handle-motif): Read targets from the targets table
of the drag window instead of the selection's TARGET target.