Allow custom shortcut for Previous Tab / Next Tab

[sitaktif]

Keyboard shortcuts can now be specified using `about:keyboard`, which is great!

Please add the `Previous Tab` / `Next Tab` command to the list of customisable shortcuts.

There is a related issue tracking the addition of the `Previous Tab` / `Next Tab` commands to the top menu, allowing users to set shortcuts for it on some OSes (e.g. macOS), but this issue is specifically to track the addition of those to `about:keyboard`.

[Jon]

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