Start a drag carrying paths, out of this app and into whatever
application the pointer is released over. Call from a press-and-move
gesture once the pointer has travelled DRAG_START_THRESHOLD_PX while
the button is still held, so the compositor can validate the drag against
that implicit grab.
The text/uri-list body for paths: one file:// URI per line, CRLF
terminated as RFC 2483 wants, with the bytes a URI cannot carry
verbatim percent-encoded. A path that is not absolute has no file URI and
is left out.