Implement `Add Entry to route file` quick assist.

Offer 'add entry to route file' for methods that return `play.api.mvc.Action` as
a quick-assist (`CMD-1` on method name).

Also:

* changed the icon for route files
* associate the route editor with sub-project routes (`*.routes`)

Fixed #85, #87.
Merge pull request #92 from dragos/issue/quick-assist-scala-85

Implement `Add Entry to route file` quick assist.