Sharpen ActionBrowseContext logic

The resolution of ActionBrowseContexts has been a bit fuzzy. Try to sharpen it a bit per the following spec

• ActionBrowseContext holds all action contributions related to given set of browse contexts.
• There are indirect (type index lookup) and direct ways (relation, direct or asserted) to contribute browse contexts for a resource.
• Additionally Model Browser merges in all browse contexts encountered along the path of the NodeContext