[Interactions] Support keybindings / keyboard handlers

Currently, we only support interactions through mouse-policies. We should support keyboard interactions, with at least two mechanisms:

- GEF Key Handlers (IOnStrokeHandler), for Diagram interactions (Or modifiers, to be used with Mouse Interactions)
- Eclipse Key Bindings, for standard Command/Handler interactions (e.g. Deleting an element)

In most cases, we want Eclipse Key Bindings, as they can be integrated with Menus or Toolbars as well; but we should provide examples for both cases.

In Diagrams, I think Papyrus only uses Keyboard as Mouse-interaction modifiers (All keybindings are delegated to the Eclipse Commands framework).

The issue names no files or tests. Start by tracing the existing mouse-policies and Diagram interaction entry points, then compare GEF IOnStrokeHandler with Eclipse Key Bindings and define examples for both, including a standard command such as deleting an element.

Written by the indexing model from the issue text.