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/main.cpp

@@ -100,7 +100,7 @@ int main(int argc, char** argv)
     if (parser.isSet(debugInfoOption)) {
         QTextStream out(stdout, QIODevice::WriteOnly);
         QString debugInfo = Tools::debugInfo().append("\n").append(Crypto::debugInfo());
-        out << debugInfo << endl;
+        out << debugInfo << Qt::endl;
         return EXIT_SUCCESS;
     }
 
@@ -194,7 +194,7 @@ int main(int argc, char** argv)
             // we always need consume a line of STDIN if --pw-stdin is set to clear out the
             // buffer for native messaging, even if the specified file does not exist
             QTextStream out(stdout, QIODevice::WriteOnly);
-            out << QObject::tr("Database password: ") << flush;
+            out << QObject::tr("Database password: ") << Qt::flush;
             password = Utils::getPassword();
         }
         mainWindow.openDatabase(filename, password, parser.value(keyfileOption));