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

tests/unit_tests/test_generate_cli_client.py

@@ -70,7 +70,7 @@ def test_client_generator_with_black_formatting():
         import enum
         from typing import Literal, Optional, overload
 
-        from bec_widgets.cli.client_utils import BECGuiClientMixin, RPCBase, rpc_call
+        from bec_widgets.cli.client_utils import RPCBase, rpc_call
 
         # pylint: skip-file