refactor: BECGuiClientMixin -> BECGuiClient

- Mixin class was only used with BECDockArea, now it is a class by itself
which represents the client object connected to the GUI server ; ".main"
is the dock area of the main window
- Enhanced "wait_for_server"
- ".selected_device" is stored in Redis, to allow server-side to know
about the auto update configuration instead of keeping it on client

bec_widgets/cli/generate_cli.py

@@ -35,7 +35,7 @@ from __future__ import annotations
 import enum
 from typing import Literal, Optional, overload
 
-from bec_widgets.cli.client_utils import RPCBase, rpc_call, BECGuiClientMixin
+from bec_widgets.cli.client_utils import RPCBase, rpc_call
 
 # pylint: skip-file"""
 
@@ -84,7 +84,7 @@ class Widgets(str, enum.Enum):
         # Generate the content
         if cls.__name__ == "BECDockArea":
             self.content += f"""
-class {class_name}(RPCBase, BECGuiClientMixin):"""
+class {class_name}(RPCBase):"""
         else:
             self.content += f"""
 class {class_name}(RPCBase):"""