Handle mouse back button for navigation

[micahmo]

This PR adds support for navigating backwards using the mouse back button.

It's obviously hard to demo, but if you have a mouse like this...

...the back button should now navigate backwards in the app, including navigating out of the video player.

Notes

• The majority of the inspiration for this implementation came from this Flutter issue and specifically this comment.
• Per the GitHub issue, this implementation should work on Windows (where I tested it) and macOS, but not Linux due to a Flutter engine limitation,
• I didn't add any platform handling because I think this should be harmless to run on mobile. The gesture detector just won't fire.
• Feel free to review with whitespace off.

[micahmo]

Hey @edde746, I noticed you added some commits to this PR/branch. They don't seem to be related to the feature, so I'm guessing it was a mistake. I can rebase and make sure I'm up-to-date with main, but I wanted to check with you first in case you need the contents of any of these commits (since they don't appear to be in main).