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]

Thanks for submitting an idea to the Mozilla Connect community! Your idea is now open to votes (aka kudos) and comments.

[siffemcon]

While waiting for implementation,  consider the Switch To Previous Active Tab & Reload All Tabs (https://addons.mozilla.org/addon/switch-to-previous-active-tab/) add-on.