Disable printers that are not host of a group

CURA-6449

 - Group printers into hosts and non-hosts and of unknown type.
 - Show available/connectable printers first in the list.
 - Show sections for connectable and non-connectable printers.

cura/Machines/Models/DiscoveredPrintersModel.py

@@ -66,12 +66,23 @@ class DiscoveredPrinter(QObject):
 
     @pyqtProperty(bool, notify = machineTypeChanged)
     def isUnknownMachineType(self) -> bool:
-        return self.readableMachineType.lower() == "unknown"
+        return self.readableMachineType == catalog.i18nc("@label", "Unknown")
 
     @pyqtProperty(QObject, constant = True)
     def device(self) -> "NetworkedPrinterOutputDevice":
         return self._device
 
+    @pyqtProperty(bool, constant = True)
+    def isHostOfGroup(self) -> bool:
+        return getattr(self._device, "clusterSize", 1) > 0
+
+    @pyqtProperty(str, constant = True)
+    def sectionName(self) -> str:
+        if self.isUnknownMachineType or not self.isHostOfGroup:
+            return catalog.i18nc("@label", "The printer(s) below cannot be connected because they are part of a group")
+        else:
+            return catalog.i18nc("@label", "Available network printers")
+
 
 #
 # Discovered printers are all the printers that were found on the network, which provide a more convenient way
@@ -90,7 +101,7 @@ class DiscoveredPrintersModel(QObject):
     @pyqtProperty(list, notify = discoveredPrintersChanged)
     def discoveredPrinters(self) -> List["DiscoveredPrinter"]:
         item_list = list(x for x in self._discovered_printer_by_ip_dict.values())
-        item_list.sort(key = lambda x: x.device.name)
+        item_list.sort(key = lambda x: (int(not x.isUnknownMachineType), getattr(x.device, "clusterSize", 1), x.device.name), reverse = True)
         return item_list
 
     def addDiscoveredPrinter(self, ip_address: str, key: str, name: str, create_callback: Callable[[str], None],

resources/qml/PrinterSelector/MachineSelectorButton.qml

@@ -53,7 +53,7 @@ Button
                 verticalCenter: parent.verticalCenter
             }
             text: machineSelectorButton.text
-            color: UM.Theme.getColor("text")
+            color: enabled ? UM.Theme.getColor("text") : UM.Theme.getColor("small_button_text")
             font: UM.Theme.getFont("medium")
             visible: text != ""
             renderType: Text.NativeRendering

resources/qml/WelcomePages/AddNetworkPrinterScrollView.qml

@@ -71,6 +71,10 @@ Item
                 anchors.fill: parent
                 model: CuraApplication.getDiscoveredPrintersModel().discoveredPrinters
 
+                section.property: "modelData.sectionName"
+                section.criteria: ViewSection.FullString
+                section.delegate: sectionHeading
+
                 Component.onCompleted:
                 {
                     // Select the first one that's not "unknown" by default.
@@ -84,6 +88,23 @@ Item
                     }
                 }
 
+                Component
+                {
+                    id: sectionHeading
+
+                    Label
+                    {
+                        anchors.left: parent.left
+                        anchors.leftMargin: UM.Theme.getSize("default_margin").width
+                        height: UM.Theme.getSize("setting_control").height
+                        text: section
+                        font: UM.Theme.getFont("default")
+                        color: UM.Theme.getColor("small_button_text")
+                        verticalAlignment: Text.AlignVCenter
+                        renderType: Text.NativeRendering
+                    }
+                }
+
                 delegate: MachineSelectorButton
                 {
                     text: modelData.device.name
@@ -93,7 +114,7 @@ Item
                     anchors.rightMargin: UM.Theme.getSize("default_margin").width
                     outputDevice: modelData.device
 
-                    enabled: !modelData.isUnknownMachineType
+                    enabled: !modelData.isUnknownMachineType && modelData.isHostOfGroup
 
                     printerTypeLabelAutoFit: true