[Interactions] Support Context Menus in Diagrams

[CamilleLetavernier]

We currently can't open a context menu in a GEFx Diagram. We should implement this.

There are a few options for this:

• Use the Eclipse Context Menus (Via the Eclipse Menu Service), and open an SWT Menu on top of the Diagram. This would be the simplest solution
• Use the Eclipse Menu Service with a custom (JavaFX) menu. This would be more portable. However, JavaFX Context Menus in an FXCanvas don't work on Linux (Plain JavaFX Context Menus work fine, and FX-in-SWT ContextMenus work fine on other platforms as well)

We could also go for a complete custom approach, or hide the Eclipse Menu Service behind our own API to allow deployment outside of Eclipse (But in general we want to reuse the Eclipse capabilities for defining menus, so we'll probably still rely on the Eclipse Menu Service)

[mwienand]

FWIW, we plan to provide context menu abstractions for GEF as well: https://bugs.eclipse.org/bugs/show_bug.cgi?id=508985

If you have any input w.r.t. a desirable solution, feel free to let us know :D