Obtaining MT connection status

The [IsConnectedToMainTerminal] method has been added to Api V8 (https://syrve.github.io/front.api.sdk/v8/html/M_Resto_Front_Api_IOperationService_IsConnectedToMainTerminal.htm) to obtain connection status to the Main Terminal.

This method is an alternative to the notification ConnectionToMainTerminalChanged. It is designed for cases when plugins do not have time to activate by the moment a notification about connecting the terminal to the GT is received, so that they can independently monitor the connection status.