Allow plugin authors to inject actions

[simianhacker]

Plugin authors need a method for injecting actions into the discover, visualize and dashboard applications in Kibana. One idea would be to create a registry and allow authors to specify a priority for each link. Then sort the links by priority. If there are more then 5 links show 4 links and the remainder in a "More Actions" pulldown.

[rashidkpc]

Dupe of #5198