…encies

Move handler creation logic from AbstractGraph to SelectionCellsHandler plugin
where it's actually used. This prevents automatic inclusion of VertexHandler
and EdgeHandler classes (~63kB) when the plugin isn't being used (for example,
in applications that perform Graph visualization only, without Cell section).

ADDED:
- Support for registering additional edge handlers beyond the built-in ones

BREAKING CHANGES:
- Removed AbstractGraph.createHandler methods, now in SelectionCellsHandler
- Handler customization now uses composition instead of inheritance, by using
  configureVertexHandler() and configureEdgeHandler() instead of extending Graph