Fix all Qt 5.15 deprecation warnings (#7783)

* Deprecated qSort() -> std::sort()
* Replace QDateTime::toString(Qt::DefaultLocaleShortDate) with Clock::toString()
* Replace QDateTime::toString(Qt::SystemLocaleShortDate) with QLocale::system().toString(..., QLocale::ShortFormat)
* Use QDateTime::startOfDay() instead of QDate(QDateTime)
  Note: QDateTime::startOfDay() is only available in Qt 5.14, we need to guard it
* Replace QString::SkipEmptyParts with Qt::SkipEmptyParts
  Note: Its designated replacement, Qt::SplitBehavior, was only added in Qt 5.14.
* Don't call deprecated QFlags(nullptr) constructor
* QSet::{toList->values}
* Replace QList::toSet, QSet::fromList with Tools::asSet()
* QHash::insertMulti -> QMultiHash::insert
* QProcess::startDetached: non-deprecated overload
* QProcess::{pid->processId}
* QPainter::{HighQuality->}Antialiasing
* QPalette::{background->window}()
* Use Qt::{Background,Foreground}Role
* endl -> Qt::endl, flush -> Qt::flush
* Make YubiKey::s_interfaceMutex non-recursive
* OpenSSHKeyGenDialog: use non-deprecated QComboBox::sizeAdjustPolicy setting

src/core/Clock.cpp

@@ -16,6 +16,8 @@
  */
 #include "Clock.h"
 
+#include <QLocale>
+
 QSharedPointer<Clock> Clock::m_instance;
 
 QDateTime Clock::currentDateTimeUtc()
@@ -78,10 +80,14 @@ QDateTime Clock::parse(const QString& text, const QString& format)
     return QDateTime::fromString(text, format);
 }
 
-Clock::~Clock()
+QString Clock::toString(const QDateTime& dateTime)
 {
+    static QLocale locale;
+    return locale.toString(dateTime, QLocale::ShortFormat);
 }
 
+Clock::~Clock() = default;
+
 Clock::Clock()
 {
 }