Ease of use; each backend-plugin can now provide additional settings.

Any overriding cals that does so, should just add 'self.definition_file_paths' to load their additional settings. However, at the moment of this commit, you should still suppy setting_definitions_appender in the list of __init__.register for the plugin.

part of CURA-10722

cura/BackendPlugin.py

@@ -4,10 +4,10 @@ import subprocess
 from typing import Optional, List
 
 from UM.Logger import Logger
-from UM.PluginObject import PluginObject
+from UM.Settings.AdditionalSettingDefinitionAppender import AdditionalSettingDefinitionsAppender
 
 
-class BackendPlugin(PluginObject):
+class BackendPlugin(AdditionalSettingDefinitionsAppender):
     def __init__(self) -> None:
         super().__init__()
         self.__port: int = 0