Restore SvStream::PutBack(char)

It was removed as unused in a3b0ee88be3c6bff94ec6fe908f209c1dbb9748d
but the declaration was accidentally left in the <tools/stream.hxx>.
Now I need it, so restore the implementation.

Change-Id: I7da5bf74e6c8ac8375273db0dda267d1dec2ac14
Handle broken RTF files that have multiple top-level (\rtf) groups

There exists in the wild RTF files that have two top-level groups
(starting with the \rtf control word). Ignore the group-end '}' and
the following group-start '{' in that case. Horrible hack, yes. But
Word does not seem to require correctly formed RTF either, it handles
this kind of files. So we should too. At least in this case of
brokenness.

Change-Id: Idd26a82f1da1e1f873c158367469eca7d9b0d652