Improve login experience

[ShadowApex]

Currently the plugin login experience isn't the best. There are a couple of things we could do to improve this.

1. If the user has logged into steam before, we can retrieve their username from ~/.local/share/Steam/config/config.vdf. Upon plugin startup, we can check for a user account from here and try logging in.
2. Show a modal dialog window that is above the keyboard so it does not get blocked.
3. Only show the username and try to login. If login fails, ask for a password. If password fails, ask for 2FA authorization.

[hphilm]

An idea if QR support will be given at some point?