CommonClient: fix bug when using Connect button without a disconnect

[qwint]

What is this fixing or adding?

makes the kivy connect button do the same username forgetting that /connect does to fix an issue where losing connection would make you unable to connect to a different server

How was this tested?

With two servers that have different valid slot names

• connect to server1, enter slot name when prompted
• close server1 to simulate the server losing connection for any reason
• enter server2 server address (with just host:port), click Connect button
• notice without the fix the connection is rejected for invalid slot (even without a prompt)
• notice with the fix the client prompts for slot name and can connect without issue after inputting it

If this makes graphical changes, please attach screenshots.

[qwint]

Maybe it could be considered as another bug (or not a bug at all), but I see that ctx.connect also set ctx.password. Should it also set to None?

Anyway, given this at least fix the issue when there is no password, LGTM.

invalid passwords trigger a different workflow where you can fix it, nothing (in that specific workflow) for clearing and fixing a slot, so I think that's fine