I've added all of valid PlaceHolder types from
http://www.schemacentral.com/sc/ooxml/t-p_ST_PlaceholderType.html

Change-Id: I038fe43ff83699f92ff5eb9945bce12540058478
Reviewed-on: https://gerrit.libreoffice.org/6814Reviewed-by: Muthu Subramanian K <muthusuba@gmail.com>
Tested-by: Muthu Subramanian K <muthusuba@gmail.com>