Speeddial: decode initialScript data as Utf8

Summary:
BUG: 406095

Solution based on:
https://developer.mozilla.org/en-US/docs/Web/API/WindowBase64/Base64_encoding_and_decoding#Solution_4_%E2%80%93_escaping_the_string_before_encoding_it

Test Plan: Add webpage with title which contains Utf8 character (e.g. ľščťžýáíéúäô) into speeddial

Reviewers: #falkon, drosca

Reviewed By: #falkon, drosca

Subscribers: falkon, drosca

Tags: #falkon

Differential Revision: https://phabricator.kde.org/D27081