Add ability to manage multiple custom filter lists

Background / User story

After introduction of the new options page users are no more able to manage multiple Filter Groups in Firefox and their old Filter Groups are merged into one Custom Filter List in CREATE AND EDIT YOUR FILTER LIST section.

This was also requested while ago for new options page, but wasn't priority as at that point we were not under the pressure to miss the feature in the Firefox.

See more on Trac ticket #5025

What to change

• Design: TBD
• Research: TBD
• Spec: TBD

Hints for testers

TBD

Hints for translators

TBD

Integration notes

TBD