sw HTML export: handle embedded text documents

Change-Id: Ib7e55e3c82b1a1115335b68c317f5fcb084f6d28
sw HTML export: avoid invalid output for embedded documents

When an sw document is embedded inside an sw one, then the inner sw HTML export
should just write what's inside the <body>. Add a filter option for that in sw,
similar to the one already existing in sc.

Change-Id: Id2cef966c1022ba45a3540bff234029c1d396778