Outline plugin should differentiate by visibility and member type

I'm documenting a vala project and I realised that the reason why outline has been so useless to me is that when I'm using outline to review the code of a class I've been trying to understand an instance of I just need the public members not the internal stuff but there's no clear way to understand what's public or private from the outline plugin. I had a few ideas about how to fix this but it's a design task, and I don't really do mockups.

- icons: most aesthetic, requires excessive design
- collapsible tree for public and private: gets stuff out of the way
- format the labels in UML: quick, easy, unambiguous, not beginner friendly

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Start by reviewing the outline plugin and the issue's three proposed approaches: icons, collapsible public/private groups, and UML-style labels. Resolve the design direction first; done means the outline clearly differentiates members by visibility and member type, with the chosen presentation agreed by the project.

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