Port to non-deprecated QLabel::pixmap()

Quoting from:
https://github.com/qt/qtbase/commit/714409b23ce5ec33e56adce5ba1966aad67c3b34

```
The QLabel changes to the pixmap/picture getters provide the following
migration path:

QPixmap *ppix = l->pixmap(); // up to 5.15, warns in 5.15
QPixmap pval = l->pixmap(Qt::ReturnByValue); // new in 5.15, works in 6
QPixmap pixmap = l->pixmap(); // from Qt 6 on
```

Since we can't require 5.15 yet in dolphin, the port is done only when
building with Qt >= 5.15