Preserve HTML attributes recursively

For nodes that handle their own children (as well as leaf nodes and
meta items), store the first child's attributes in html/0-0/*, the
second child's attributes in html/0-1/*, the second element's third
child's fourth child's attributes in html/1-2-3/* , etc.

This obsoletes the ad-hoc code that basically did the same thing in
MWInlineImageNode.

Change-Id: If5abd2d5d9c361b359617ff4b0f3d6ba4c9b0142
Merge "Preserve HTML attributes recursively"