Fix binding loop in action panel widget

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.

plugins/PrepareStage/PrepareMain.qml

@@ -1,4 +1,4 @@
-//Copyright (c) 2019 Ultimaker B.V.
+//Copyright (c) 2020 Ultimaker B.V.
 //Cura is released under the terms of the LGPLv3 or higher.
 
 import QtQuick 2.4

resources/qml/ActionPanel/ActionPanelWidget.qml

@@ -1,4 +1,4 @@
-// Copyright (c) 2019 Ultimaker B.V.
+// Copyright (c) 2020 Ultimaker B.V.
 // Cura is released under the terms of the LGPLv3 or higher.
 
 import QtQuick 2.7
@@ -15,7 +15,6 @@ import Cura 1.0 as Cura
 Item
 {
     id: base
-    width: childrenRect.width
     height: childrenRect.height
     visible: CuraApplication.platformActivity
 

resources/qml/PrintSetupSelector/Recommended/RecommendedSupportSelector.qml

@@ -1,4 +1,4 @@
-// Copyright (c) 2018 Ultimaker B.V.
+// Copyright (c) 2020 Ultimaker B.V.
 // Cura is released under the terms of the LGPLv3 or higher.
 
 import QtQuick 2.10