Added:
- option to change tab focus to an adjacent tab instead of following the active tab
- option to move tabs before the target tab instead of after

Changed:
- marked tabs in the context menu are now sorted in tab order (instead of the order in which they were marked)
- improved Remove Teleport Target icon

Firefox 60+ is required; the old menu code to support previous versions has been removed.

Bugs fixed (thanks to Simon Pratt for finding these):
- Moving a tab to the position after itself no longer moves the tab.
- Moving a tab to the far left when there are pinned tabs now works.

Plus these changes:
- Pinned tabs can now be moved consistently.
- Tab targets will not disappear after an extension upgrade.
- If a tab target cannot be marked visually because it is unloaded, the mark will be added when it is next loaded.
- Context menu building is improved, particularly in Firefox 60+ where it no longer needs to be rebuilt as often and "Remove Teleport Target" gets its own icon.