So that it's possible to do ad-hoc dumping from gdb with something like:

print xMetaFile->dumpAsXml(0)

Similar to how that already worked for SwDoc or SwFrame.

Change-Id: I9cb884491f2a49afdb06179d8d54252101f37bb7
Reviewed-on: https://gerrit.libreoffice.org/69253Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins