Move backend plugin logic to Cura from Uranium

I was running into abstraction issues when it was defined in Uranium.
Instead of trying to fight those, it's just easier to move it to Cura

CURA-10717

UM/Application.py

@@ -34,7 +34,6 @@
     from UM.Backend.Backend import Backend
     from UM.Settings.ContainerStack import ContainerStack
     from UM.Extension import Extension
-    from UM.BackendPlugin import BackendPlugin
 
 
 @signalemitter
@@ -199,7 +198,6 @@ def initialize(self) -> None:
         i18nCatalog.setApplication(self)
 
         PluginRegistry.addType("backend", self.setBackend)
-        PluginRegistry.addType("backend_plugin", self.addBackendPlugin)
         PluginRegistry.addType("logger", Logger.addLogger)
         PluginRegistry.addType("extension", self.addExtension)
         PluginRegistry.addType("file_provider", self.addFileProvider)
@@ -481,12 +479,6 @@ def addExtension(self, extension: "Extension") -> None:
     def getExtensions(self) -> List["Extension"]:
         return self._extensions
 
-    def addBackendPlugin(self, backend_plugin: "BackendPlugin") -> None:
-        self._backend_plugins.append(backend_plugin)
-
-    def getBackendPlugins(self) -> List["BackendPlugin"]:
-        return self._backend_plugins
-
     def addFileProvider(self, file_provider: "FileProvider") -> None:
         self._file_providers.append(file_provider)
 

UM/Backend/Backend.py

@@ -54,7 +54,6 @@ def __init__(self) -> None:
 
         self._socket = None
         self._port = 49674
-        self._last_backend_plugin_port = self._port + 1000
         self._process: Optional[subprocess.Popen] = None
         self._backend_log: List[bytes] = []
         self._backend_log_max_lines = None
@@ -77,20 +76,6 @@ def setState(self, new_state: BackendState) -> None:
             if self._backend_state == BackendState.Done:
                 self.backendDone.emit()
 
-    def startPlugins(self) -> None:
-        """
-        Ensure that all backend plugins are started
-        :return:
-        """
-        backend_plugins = UM.Application.Application.getInstance().getBackendPlugins()
-        for backend_plugin in backend_plugins:
-            if backend_plugin.isRunning():
-                continue
-            # Set the port to prevent plugins from using the same one.
-            backend_plugin.setPort(self._last_backend_plugin_port)
-            self.__last_backend_plugin_port += 1
-            backend_plugin.start()
-
     def startEngine(self) -> None:
         """
         Start the backend / engine.