org-odt.el: Create and update manifest.xml towards end of export

org-odt.el: Create and update manifest.xml towards end of export

* contrib/lisp/org-odt.el (org-export-odt-manifest-lines):
Removed.
(org-odt-update-manifest-file): Removed.
(org-odt-create-manifest-file-entry)
(org-odt-write-manifest-file): New function.
(org-odt-manifest-file-entries): New variable.
(org-odt-manifest-file-entry-tag): Updated.
(org-odt-init-outfile): Don't create manifest.xml here.  Don't
create Pictures directory so early in the export.  Reset
`org-odt-manifest-file-entries'.
(org-odt-save-as-outfile): Modified.  Use
`org-odt-create-manifest-file-entry' and
`org-odt-write-manifest-file'.
(org-odt-copy-image-file, org-odt-update-meta-file)
(org-odt-copy-styles-file): Use
`org-odt-create-manifest-file-entry'.

Rationalized creation of Manifest file entries.  New entries are
created right at the point where new files are added to the odt file.