Refactors TabView using modern v2 capabilties

[tig]

TabView is a great control for testing the library. It has a lot of capabilities that stretch the library. The v1 version was internally very complex in order to work around limitations in TG. In theory, v2 should enable making this control a lot simpler.

This PR is a WIP/Prototype that I started to determine if we're on the right track with these v2 capabilities:

• Content scrolling - The TabRowView can/should just be a horizontally scrolling View where each Tab is just a subview. All the complex TabView logic for renering the Tabs and supporting scrolling can go away.
• Layout - Dim.Auto, deferred layout, etc... should reduce the need for a lot of the internal complexity. Especially the complex layout math scattered around.
• Drawing - There should be no need for TabView to draw lines directly. Each element can just be a View and leverage Border an automatic line joins currently enabled by SuperViewRendersLineCanvas
• Navigation - How a user navs with the keyboard across Tabs, TabRowView, and the tab content is complex. The new v2 Nav system should make this easier.

Depends on

• Fixes #3761, #2886 - Draw and Layout performance issues #3798

In #3798 auto-line join across Views is currently broken. I'm pausing work on this PR until I fix that.

Fixes

Proposed Changes/Todos

• Prototype TabRowView that just uses content scrolling
• Remove ContentView
• Remove all complex LineCanvas stuff and just rely on Border

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
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