Resolve "Multiple windows and dock areas from client"

Description

This MR does the following:

• refactoring of BECGuiClientMixin -> it is now renamed BECGuiClient and is not a mixin class anymore
  • the gui object in CLI is an instance of this class
  • it gives access to the main dock area in the main window (.main)
  • as a consequence BECDockArea becomes like all other client-side objects (just inheriting RPCBase)
  • better handling of timeout if GUI server is not present
  • no need for Proxy anymore on the BEC startup/CLI client side
• gui.new(name) -> returns a new BECDockArea as an independent window
  • windows can be inspected with gui.window dictionary
• .selected_device is now stored in Redis, server-side can retrieve it (or can receive notifications) for Auto Update feature
  • instead of having this information in client only
• AutoUpdates code benefits from the generator executor in RedisConnector (see corresponding MR in BEC)
• gui.show(), gui.hide() allow to show or hide all top-level windows
• gui.close(), gui.start() allow to stop/restart the GUI server
  • start() is asynchronous, do .start_server(wait=True) for the synchronous version
• BECDockArea has a .delete() method, which also cleans itself from gui.window
• RPC-related classes are moved to a dedicated rpc sub-directory

Closes #422 (closed)