Make it possible to track selected nodes across matching (Matcher::match()).

A possible solution is to introduce a IGV-specific node identifier (decoupled from Node::_idx) that remains the same after renumbering or matching a node. While this solution cannot totally capture the many-to-many relation induced by matching, it is simple, it might be "good enough" in practice (captures most of the interesting node relations before and after matching), and is never worse than the current behavior (where no relation is captured anyway).

The attached screenshots illustrate the potential effect of this enhancement. before-matching.png shows a graph corresponding to the "Before matching" phase, where node (130 AddI) is extracted. after-matching.png shows the desired graph after transitioning to the "After matching" phase, where the node (24 incI_rReg), the replacement of (130 AddI) by matching, is show instead. The current behavior is that no node is shown in the best case, and a totally unrelated node is shown if there is a node identifier collision (see JDK-8264842).