This actually implements the reading of the VBA stream in Impress too, so the
user will get a warning when opening such a file (that the it contains
macros).

Change-Id: I1638199529196ed217cbd9ebef88eb7c88f2179a
Reviewed-on: https://gerrit.libreoffice.org/40197Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>