GUI becomes unresponsive during connection issues

[Rob-71]

Describe the bug
When connecting to a new session that does not respond for any reason, the entire superxputty application stops responding until the session finally responds by either connecting or failing. In some cases this can leave superxputty unresponsive for a significant amount of time.

To Reproduce
Steps to reproduce the behavior:

1. Open any existing connection to a host which is responding very slowly.
2. Try to perform any activity in superxputty menus, open another session or switch tab to an existing session.

Expected behavior
The GUI controls and existing tabs should remain responsive even if a single session is not responding for any reason.