Only show Machine Actions for the currently active machine

Many machine actions act on the currently active printer, because they don't have a pointer to the printer selected on the Printer manager.

resources/qml/Preferences/MachinesPage.qml

@@ -74,6 +74,7 @@ UM.ManagementPage
         Flow
         {
             id: machineActions
+            visible: currentItem && currentItem.id == Cura.MachineManager.activeMachineId
             anchors.left: parent.left
             anchors.right: parent.right
             anchors.top: machineName.bottom