Refactor CClientDlgOnConnectDisconnectBut()

[ann0see]

          This should be implemented by switching the `connect` setting for the signal handler for the button, rather than having one signal handler with an if that consults the state of the client.

Button life-cycle:

• (public) ConnectDlg onClickConnect is connected to (private) ClientDlg onConnectDlgConnect
• onClickConnect launches ConnectDlg if it's not already open (and nothing else)
• onConnectDlgConnect disables the button and asks the client to connect to the provided address
• onConnect switches the onClick handler to onClickDisconnect and enables the button
• onConnectFailed just enables the button (keeping the onClickConnect handler) -- although, if Client is passing an error, here's where it would get displayed to the user
• onClickDisconnect disables the button and asks the client to disconnect
• onDisconnect (which should be handled) switches the onClick handler to onClickDisconnect and enables the button

Originally posted by @pljones in #3372 (comment)