Don't allow controlling frame index from window renderer

[bbondy]

This mainly just reacts to index changes whenever they happen instead of controlling the window renderer index from tab moves.

It also simplifies the code a bit in terms of internal index which is the same as index on the window renderer.

Fix #10961

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[bbondy]

0.19.x: 5f76327

0.20.x: 49426f2

master: 49426f2