Nikolai Krasheninnikov authored Mar 22, 2020 and Elvis Angelaccio committed Mar 22, 2020

Summary:
1) Pointers initialized with 'nullptr' instead of '0'.
2) Moved to a modern Qt-way of connecting signals and slots without SIGNAL() and SLOT() macros.
3) Class SVNCommands renamed to SvnCommands for consistency with other class names.

Test Plan: Ensure everything is correct, try some basic actions.

Reviewers: meven, elvisangelaccio

Differential Revision: https://phabricator.kde.org/D28170