Add "refresh" button to list of network printers

CURA-2035

DiscoverUM3Action.py

@@ -23,6 +23,9 @@ class DiscoverUM3Action(MachineAction):
             self._network_plugin.addPrinterSignal.connect(self._onPrinterDiscoveryChanged)
             self._network_plugin.removePrinterSignal.connect(self._onPrinterDiscoveryChanged)
             self.printersChanged.emit()
+        else:
+            # Restart bonjour discovery
+            self._network_plugin.startDiscovery()
 
     def _onPrinterDiscoveryChanged(self, *args):
         self.printersChanged.emit()

DiscoverUM3Action.qml

@@ -45,6 +45,8 @@ Cura.MachineAction
     {
         anchors.fill: parent;
         id: discoverUM3Action
+        spacing: UM.Theme.getSize("default_margin").height
+
         SystemPalette { id: palette }
         UM.I18nCatalog { id: catalog; name:"cura" }
         Label
@@ -61,7 +63,16 @@ Cura.MachineAction
             id: pageDescription
             width: parent.width
             wrapMode: Text.WordWrap
-            text: catalog.i18nc("@label", "To print directly to your Ultimaker 3 printer over the network, please make sure your printer is connected to the network using a network cable or by connecting your printer to your WIFI network. \n\nIf you don't want to connect Cura with your Ultimaker 3 now, you can always use a USB drive to transfer g-code files to your printer.\n\nSelect your Ultimaker 3 from the list below:")
+            text: catalog.i18nc("@label", "To print directly to your Ultimaker 3 printer over the network, please make sure your printer is connected to the network using a network cable or by connecting your printer to your WIFI network. If you don't connect Cura with your Ultimaker 3, you can still use a USB drive to transfer g-code files to your printer.\n\nSelect your Ultimaker 3 from the list below:")
+        }
+
+        Button
+        {
+            id: rediscoverButton
+            text: catalog.i18nc("@title", "Refresh")
+            onClicked: manager.startDiscovery()
+            anchors.right: parent.right
+            anchors.rightMargin: parent.width * 0.5
         }
 
         Row

NetworkPrinterOutputDevicePlugin.py

@@ -29,6 +29,15 @@ class NetworkPrinterOutputDevicePlugin(OutputDevicePlugin):
 
     ##  Start looking for devices on network.
     def start(self):
+        self.startDiscovery()
+
+    def startDiscovery(self):
+        if self._browser:
+            self._browser.cancel()
+            self._browser = None
+            self._printers = {}
+        self._zero_conf.__init__()
+
         self._browser = ServiceBrowser(self._zero_conf, u'_ultimaker._tcp.local.', [self._onServiceChanged])
 
     ##  Stop looking for devices on network.