[Feature Request] Insert new related tab at the: «Top (after the last unread tab)»

[myrdd]

First, thanks a lot @traxium for making and maintaining this add-on! I'd like to request a new option to the «Insert new related tab at …» preference

Steps to reproduce:

1. Set option „Insert new related tab at the“ to „Top“
2. Open a link in a foreground tab (e.g. ctrl-shift-click), “child tab 1”
3. Switch back to the original tab
4. Open a second link in a new background tab (e.g. ctrl-click), “child tab 2”
5. Open a third link in a new tab, “child tab 3”
   1. either in background (e.g. ctrl-click)
   2. or in foreground (e.g. ctrl-shift-click)

What happens? The third tab (child tab 3) opens as the first child. The final list of child tabs is in this order:

• child tab 3 – third link – unread (in case of step 5.i) or selected
• child tab 2 – second link – unread
• child tab 1 – first link – read

What should happen? I'd like to have the third tab (child tab 3) opened as the second child, that is, the list of child tabs is:

• child tab 2 – second link – unread
• child tab 3 – third link – unread (in case of step 5.i) or selected
• child tab 1 – first link – read

More generally, new tabs should be placed at the top of the child list, but if there are some unread tabs at the top, the new tab should be placed directly after the bottom-most unread tab. This would be similar to Firefox' default behavior.

This behavior could be provided via a new option, or could replace the current behavior of the “insert at the Top” option