Fix various tab bugs and add tests.

[texodus]

This PR fixes various tab bugs and cleans up the implementation, as well as adding a few new test suites.

• Fixes several conditions where the tablist could become "stale" (typically due to tab switching, then back).
• Fixed tab callback effects becoming stale (e.g. triggering the wrong tab).
• Fixes overlay implementation to just be grid and absolute; defunct interactive mode has been removed as it is now identical to grid mode, because ...
• ... slot manipulation has been replaced by a user-definable Overlay class, which defaults to just "overlay".
• Tabs now have a mini-vdom implementation and only update when their data changes.