Support new Textual footer

[godlygeek]

Textual 0.63 introduced a new Footer widget. We need some changes to work with this:

• Because it queries the app's bindings earlier than the old footer, we need to be careful to avoid querying some widgets before they've been mounted
• The hack we were using to force the footer to redraw with new descriptions for keybindings no longer works with the new Footer, but I found another (nicer) one
• We need to update our snapshot tests to account for the new footer's style

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.86%. Comparing base (41248ed) to head (e2163c3). Report is 58 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #613 +/- ## ========================================== + Coverage 92.55% 92.86% +0.31% ========================================== Files 91 92 +1 Lines 11304 11237 -67 Branches 1581 2055 +474 ========================================== - Hits 10462 10435 -27 + Misses 837 802 -35 + Partials 5 0 -5 ``` | [Flag](https://app.codecov.io/gh/bloomberg/memray/pull/613/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg) | Coverage Δ | | |---|---|---| | [cpp](https://app.codecov.io/gh/bloomberg/memray/pull/613/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg) | `92.86% <100.00%> (+6.92%)` | :arrow_up: | | [python_and_cython](https://app.codecov.io/gh/bloomberg/memray/pull/613/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg) | `92.86% <100.00%> (-2.86%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=bloomberg#carryforward-flags-in-the-pull-request-comment) to find out more.

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