This widget can't set its width to the width of its children, since its children anchor to its right-hand side which depends on the width.
This fixes the following warning in the log:
2020-03-20 14:25:20,351 - WARNING - [MainThread] UM.Qt.QtApplication.__onQmlWarning [406]: file:///home/trin/Gedeeld/Projects/Cura/plugins/PrepareStage/PrepareMain.qml:16:5: QML ActionPanelWidget: Binding loop detected for property width
Done during Turbo Testing and Tooling.
They can then decide if it needs to be there. For instance, the monitor stage doesn't want to have this panel. Also each stage can position it where they like, and in the near future I'll make a feature to let it decide whether or not to show the preview button.
Contributes to issue CURA-6086.
