Allow machine actions to reference the dialog they are shown in

This way they can respond to the onAccepted and onRejected actions of the dialog
Contributes to CURA-2019

resources/qml/MachineAction.qml

@@ -3,6 +3,10 @@ import QtQuick 2.2
 Item
 {
     id: contentItem
+
+    // Point to the dialog containing the displayItem
+    property var dialog
+
     // Connect the finished property change to completed signal.
     property var finished: manager.finished
     onFinishedChanged: if(manager.finished) {completed()}

resources/qml/Preferences/MachinesPage.qml

@@ -106,6 +106,7 @@ UM.ManagementPage
             {
                 contents = content;
                 content.onCompleted.connect(hide)
+                content.dialog = actionDialog
             }
             rightButtons: Button
             {