Allow graph format to display some properties on graph nodes, rather than edges

[alex-mashin]

Allow graph format to display some properties on graph nodes, rather than edges.

This will be especially appropriate for properties not of the type Page.

Possible approaches are (please comment):

• adding a new parameter dot label which will be a template for dot label containging placeholders for property values. If a property in a result row has a corresponding placeholder in the label template, its value(s) is substituted there instead, and no edge and parent node is created,
• displaying all properties not of type Page in nodes (dot labels) rather than as edges and parent nodes.

[alex-mashin]

Still not merged, checks pending.