Support override markers/indicators for role classes

[stephan]

(spawning from #176):

When a role class overrides a role from its team's super-team this should be visualized in the tools just like method overriding is shown:

• outline and similar views should add an override indicator (overlay) to the role's icon
• the editor should show override markers supporting the "Open super implementation" action.

[stephan]

r19740 implements the core part for checking whether a role has one or more tsuper roles.

• IRoleType / RoleType implement a new method getTSuperRoles() and if this finds any, that fact is recorded by setting AccOverriding
• the compiler part is about retrieving AccOverriding from the corresponding bit in OT_CLASS_FLAGS

[stephan]

Screenshot showing multiple role decorations

[stephan]

Implemented in r19743:

• Define a new decorations via extension point org.eclipse.ui.decorators, use a dummy implementation, because the ViewAdaptor already implements the decoration, all we need is to register decorations for the preferences.
• ViewAdaptor uses AccOverriding (first priority because lightweight) or IRoleType.getTSuperRoles() to determin overriding.

Here's a screenshot showing off several decorations:

• roles Subscriber and Item have both an override and a bound-role decoration
• role FlightItem only has the bound-role decoration
• decorations are shown in the outline and also in the breadcrumbs
• the vertical rules shows override annotations for Subscriber and Item
• hover on such annotations says: "overrides bonussystem.Bonus.Item"

Not shown in the screenshot: the context menu on the vertical ruler offers the "Open super implementation" action, which in this case takes you directly to the overridden role "bonussystem.Bonus.Item".