use green color for graph nodes

- also use stylesheet and classes instead of directly
  injected css.
change color of edge dependent on link-type.
enhance the styling of graphs and related infos.
fix path after node selection issue

When selecting a path(edge) after a node(node)
the node stayed visually selected.
add functions to add/remove classes in svg.
show an edge as selected when it is selected.
improve highlighting of nodes when hovering.
improve drawing of edge arrowhead.
highlight nodes when hovering on pertaining edge.
display graph related notifications

If a graph has no edges we do not display it at all
and show es specific notification. As long
as we load the graph we show a graph loading
notification.
add modes to ontology-graph display

Add support for import and normal mode supported

- import: Show only links with kind: import_or_view
- normal: Show all links.
show symbols as links in graph node view.