qt: Improve capslock detection on non-us keyboards (issue #1855)

On non-us keyboards you can obtain lower case characters even pressing
the SHIFT, this caused false positives.

src/qt/askpassphrasedialog.cpp

@@ -235,7 +235,7 @@ bool AskPassphraseDialog::eventFilter(QObject *object, QEvent *event)
         if (str.length() != 0) {
             const QChar *psz = str.unicode();
             bool fShift = (ke->modifiers() & Qt::ShiftModifier) != 0;
-            if ((fShift && psz->isLower()) || (!fShift && psz->isUpper())) {
+            if ((fShift && *psz >= 'a' && *psz <= 'z') || (!fShift && *psz >= 'A' && *psz <= 'Z')) {
                 fCapsLock = true;
                 ui->capsLabel->setText(tr("Warning: The Caps Lock key is on!"));
             } else if (psz->isLetter()) {