[Qt] Use Qt text codec instead of nunicode

Remove one extra dependency. Qt has some bugs on the text codec it
seems, but it is also important to keep the behavior aligned with
the client API.

platform/qt/src/string_stdlib.cpp

@@ -0,0 +1,24 @@
+#include <mbgl/platform/platform.hpp>
+
+#include <QByteArray>
+#include <QString>
+
+#include <string>
+
+namespace mbgl {
+namespace platform {
+
+std::string uppercase(const std::string& str) {
+    auto upper = QString::fromUtf8(str.c_str()).toUpper().toUtf8();
+
+    return std::string(upper.constData(), upper.size());
+}
+
+std::string lowercase(const std::string& str) {
+    auto lower = QString::fromUtf8(str.c_str()).toLower().toUtf8();
+
+    return std::string(lower.constData(), lower.size());
+}
+
+} // namespace platform
+} // namespace mbgl