Fixed deprecation warnings

Fixed all the warnings generated by Qt-5.14.2 + GCC-9.3.0 setup
to restore the clean build.

Examples of the warnings:

• ‘QList QSet::toList() const [with T = QUrl]’ is deprecated: Use values() instead. [-Wdeprecated-declarations]
• ‘QSet QList::toSet() const [with T = QUrl]’ is deprecated: Use QSet(list.begin(), list.end()) instead. [-Wdeprecated-declarations]
• ‘static QSet QSet::fromList(const QList&) [with T = QString]’ is deprecated: Use QSet(list.begin(), list.end()) instead. [-Wdeprecated-declarations]
• implicitly-declared ‘FilterSettings::FilterSettings(const FilterSettings&)’ is deprecated [-Wdeprecated-copy]
• ‘int QTime::elapsed() const’ is deprecated: Use QElapsedTimer instead [-Wdeprecated-declarations]
• ‘void QTime::start()’ is deprecated: Use QElapsedTimer instead [-Wdeprecated-declarations]
• ‘void QTextEdit::setTabStopWidth(int)’ is deprecated [-Wdeprecated-declarations]

Discussion: https://invent.kde.org/utilities/krusader/-/merge_requests/7