Move tabs and groups per Jackson McKay

Shortcuts:
Tab: Move left - Alt + Shift + Left
Tab: Move right - Alt + Shift + Right
Group: Move left - Alt + Shift + ,
Group: Move right - Alt + Shift + .

Mac shortcuts:
Tab: Move left - Cmd + Control + Left
Tab: Move right - Cmd + Control + Right
Group: Move left - Cmd + Control + ,
Group: Move right - Cmd + Control + .

The following shortcuts also exist but don't have a default keybinding:
- Tab: Pin/Unpin
- Tab: Ungroup
- Tab: Move to the start
- Tab: Move to the end
- Tab: Move to the next window
- Tab: Move to the previous window
- Group: Move to the start
- Group: Move to the end
- Group: Move to the next window
- Group: Move to the previous window
- Group: New tab in current group

All shortcuts can be assigned/reassigned in Firefox by going to about:addons and selecting "Manage Extension Shortcuts" from the settings cog.

Notes:

If a tab runs off the edge of the screen, it wraps back around to the start.

Pinned tabs are fully supported.

If an ungrouped tab is moved around:
- with the single tab shortcuts, it will naturally join and unjoin tab groups as it runs into them
- with the tab group shortcuts, it will skip around any existing tab groups