Link-based node navigation

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The Problem

The node navigation implemented in #1199 assumes a tree structure, but Tapestries are not restricted to a tree structure. The possibility of multiple parents make the arrow-key-based node navigation a suboptimal solution. With the solution implemented in #1199, a user using just their keyboard cannot navigate to some nodes if one cannot reach that node from the root node following the direction of links.

The Solution

An alternative way of navigating the nodes in a tapestry by keyboard that follows links to go to other nodes. This includes parent-child links in any direction, or associative links that are not parent-child links (for better compatibility with the new authoring workflow).