Add DrawLine event to TreeView for advanced rendering manipulation

[tznind]

Fixes #_____ - Added DrawLine event to TreeView for advanced rendering manipulation

TreeView with files visible that uses DrawLine event to render files in yellow

Design

I have reused the RuneCell class for the line but I could instead use List<Attribute> and List<Rune>. Both feel a little janky though. RuneCell works with ColorScheme not Attribute so its not ideal. Changing to lists introduces the possibility of them going out of sync (e.g. where user changes 1 list but not the other). Finally I could change to Tuple<Rune,Attribute>. Let me know what you think

Pull Request checklist:

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