Switch to the new connect() API

Summary:
Switch to the new connect API with removal of the SIGNAL and SLOT calls

This required the following changes
- Replace the optional parameters slots by explicit ones to manage the overload conflicts
- Additional disconnects in destructors for LokalizeMainWindow and TranslationUnitTextEdit

Reviewers: #localization, aacid, ltoscano, shaforostoff, mlaurent

Subscribers: broulik

Tags: #localization

Differential Revision: https://phabricator.kde.org/D14854