Added completed signal to machine action, so model can notify display that it is completed

CURA-1385

resources/qml/MachineAction.qml

@@ -0,0 +1,10 @@
+import QtQuick 2.2
+
+Item
+{
+    id: contentItem
+    // Connect the finished property change to completed signal.
+    property var finished: manager.finished
+    onFinishedChanged: if(manager.finished) {completed()}
+    signal completed()
+}