Added machine action buttons dynamic visibility

Based on this change, users now will be able to manage the visibility of their machine action plugin implementations. For example, a machine action only visible when loggedIn.

cura/MachineAction.py

@@ -114,3 +114,15 @@ class MachineAction(QObject, PluginObject):
     @pyqtSlot(result = QObject)
     def getDisplayItem(self) -> Optional["QObject"]:
         return self._createViewFromQML()
+
+    @pyqtSlot(result = bool)
+    def isVisible(self) -> bool:
+        """Whether this action button will be visible.
+
+         Example: Show only when isLoggedIn
+
+        :return: Defaults to true to be in line with the old behaviour.
+        """
+
+        return True
+    

resources/qml/Preferences/MachinesPage.qml

@@ -67,6 +67,7 @@ UM.ManagementPage
             {
                 width: Math.round(childrenRect.width + 2 * screenScaleFactor)
                 height: childrenRect.height
+                visible: machineActionRepeater.model[index].isVisible()
                 Cura.SecondaryButton
                 {
                     text: machineActionRepeater.model[index].label