Several not-interrelated minor changes that were pending

In more detail:
    - Some adaptations due to the code style.
    - Some changes that @abika proposed in https://phabricator.kde.org/D24987.
    - Some comments were improved or added.
    - In one line: `sym link` was replaced with `symlink`, as in all the source code of Krusader.
    - A `KMessageBox::sorry()` is used the same way that it's utilized in `void ListPanelFunc::editFile(const QUrl &filePath)`, where it's written `KMessageBox::sorry(krMainWindow, i18n("You cannot edit a folder"));`.

Differential Revision: https://phabricator.kde.org/D27609