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