TS: Comment-nodes support for assertEqualsLxml

Halfway implementing this, CompareXml.xpathToHere(...) is done;
comparing a Comment-node inside the root-tag also.

TODO: support for rootless Comment-node (absence) support.
TS: finished Comment-node differences support in assertEqualsLxml
TS: added WIP-test.
TS: assertEqualsLxml support for processing-instruction's added.
TS: assertEqualsLxml now supports Entity-nodes (rare, needs parsing option to occur).
TS: first minimally-working showContext for assertEqualsLxml
TS: assertEqualsLxml's showContext now works for all inequalities.

TODO: Nasty prefix-clobbered XML results in unusable getpath() output.
      Fix by using another approach to re-find a node-instance in a new,
      (except for node.sourceline) identical tree.
TS: equal handling or Node or Tree works again
TS: removed obsolete TODO
TS: fixed coloring and marking the first-differences line

In assertEqualsLxml's showContext output.