plug-ins: fix incorrect saving of Iptc.Application2.Caption in metadata-editor.

The saved value for Iptc.Application2.Caption is copied from Xmp.dc.description.
However the last one is multiline but the former should be single line. This
caused only the first line to be saved instead of all lines.

To fix this we set Iptc.Application2.Caption to single and use a different
conversion based on whether the tags from Xmp and Iptc are both
multiline or whether Iptc is single line.

(cherry picked from commit 5d14c59d)