UI: Set default string size arg for QT_UTF8 / QString::fromUtf8

Our QT_UTF8(str) macro uses QString::fromUtf8(str) with no size
argument. In Qt5, QString::fromUtf8 uses a default value of -1 for the
size arg. If size is -1, it is taken to be strlen(str). In Qt6,
QString::fromUtf8 doesn't use a default value for the size arg, but has
the same behavior if you manually specify -1 for the size. Let's
manually specify -1 to maintain the same behavior between Qt5 and Qt6.

https://doc.qt.io/qt-5/qstring.html#fromUtf8
https://doc.qt.io/qt-6/qstring.html#fromUtf8

UI/qt-wrappers.hpp

@@ -28,12 +28,7 @@
 #include <memory>
 #include <vector>
 
-#if QT_VERSION >= QT_VERSION_CHECK(6, 0, 0)
 #define QT_UTF8(str) QString::fromUtf8(str, -1)
-#else
-#define QT_UTF8(str) QString::fromUtf8(str)
-#endif
-
 #define QT_TO_UTF8(str) str.toUtf8().constData()
 
 class QDataStream;