Add support for excluding certain active connections from multi ssh commands

Original Description:
Requesting feature to disable/enable accepting commands for "Multi ssh"

Merging request from #1100:

Send command using the MultiSSH toolbar only to the SSH connection inside the currently focused Connection Panel. Preferably optional by checkbox.

Expected Behavior

It would be nice if there would be an option to limit the command forwarding only to the SSH connections, which are in the currently focused Connection Panel.

Current Behavior

Currently the MultiSSH toolbar will send the command to all open SSH connections, independent of the Connection Panel.

Possible Solution

Add checkbox to the MultiSSH toolbar to limit command forwarding only to the SSH connection in the current Connection Panel.

Context

We have SSH connections to different Docker machines. It would save time to have all connections open but to organize them into Connection Panels and to send commands based on the panels.

Start by tracing the MultiSSH toolbar's command-forwarding path and how the currently focused Connection Panel is identified. Add an optional setting that limits forwarding to SSH connections in that panel, and verify that the default behavior still sends commands to all open SSH connections.