Fix PrivateDataResult.getChildElementXML()

that method never worked correctly since 11 years, ie. the PrivateData
was never added as element text. It's not surprisingly that this was not
discovered in more then a decade, since Smack usually never *sends*
those stanza but only receives them.

But that's no reason to not fix it. :)
Properly escape Bookmarks and FormField XML

by using XmlStringBuilder. Fixes SMACK-577

Also extend LazyStringBuilder with a cache. And extend XmlStringBuilder
with some more convenience methods.

Move the ELEMENT and NAMESPACE definition from Form to DataForm, where
it belongs.
Improve parseContent() and parseContentDepth()

The idea is that xml-roundtrip should *never* be expected from a
XmlPullParser. So what we need is a method that parses the content of an
element without relying on getText() returning text if on START_TAG or
END_TAG. This is already done by PubSubs ItemProvider.