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/keys/drivers/YubiKey.h

@@ -24,6 +24,7 @@
 #include <QMutex>
 #include <QObject>
 #include <QTimer>
+
 #include <botan/secmem.h>
 
 typedef QPair<unsigned int, int> YubiKeySlot;