Format XML string with different indents for nodes of different levels

[mstarodubtsev]

It is more convenient to see the printed XML document as the string with different indents for every level of the nodes hierarchy.
So we can get (maybe xml.toPrettyString()):

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<xml>
    <level1>
        <level2>
            <level3/>
            <level3/>
        </level2>
    </level1>
</xml>

Instead of (now xml.toString(), with default javax.xml.transform.TransformerFactory):

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<xml>
<level1>
<level2>
<level3/>
<level3/>
</level2>
</level1>
</xml>

[dmarkov]

@yegor256 dispatch this issue please, see par.21