Decided to keep the migrated/isolated Emf/Wmf reader
which are now hidden behind a Uno Api. Had to re-implement
WMF_EXTERNALHEADER (now WmfExternal, own file/header)
to not break anything. It *seems* to just scale something
and could be done after import, but I could not be sure.
Also needed a callback hook to allow getting the Metafile
out of a MetafilePrimitive in a lower module (vcl relative
to drawinglayer) which is needed as long as primitives
are not completely on Uno Api. Deleted all Emf/Wmf reader
stuff from vcl.

Change-Id: Ic5540defa8ec770728280df4df3f12e1f48cfc3a