Allow opening a view or popup directly by intent and without specifying a capability-specific payload

[danielwiehl]

It should be possible for an application to open a view or a popup directly by intent and without specifying a capability-specific payload. Both WorkbenchRouter and WorkbenchPopupService should be convenience API only.

For a standardized API between the different capability types, context-related information should optionally be passed via message headers and not be part of the payload.