Optimize outline view for cleaner presentation, easier navigation & orientation

[tedepstein]

The outline view is supposed to be useful for navigation, showing a summary of the editor's content. But the current outline view for SwagEdit is not very useful, because there's too much detail and "noise".

Some things that would help:

• Most nodes have a suffix like tag:yaml.org,2002:seq. This is difficult to read and seems like it's only intended to tell us the JSON/YAML node type. We would be better off using different icons to present this information. Different icons for each node type.
• The outline mirrors the JSON structure node-for-node, which is very nearly line-for-line. So when fully expanded, the outline view has almost the same number of lines as the YAML document. We should provide filters to control the level of detail; and/or make it easy to collapse the tree to a given level of detail with a single UI action.
• Buttons to expand all, collapse all, and toggle sync with the editor would be helpful.

[tedepstein]

Note that YEdit has an option to "Show tags in outline view" which is enabled by default. Turning it off makes the view considerably less noisy. If it's possible for SwagEdit to override this default, that would be helpful.