shell: Disable the "Next" button appropriately.

Disable the "Next" button when the address or host name is empty.
If the address or host name is incorrect, the button is disabled, but still freeze,
this will be fixed together with the returned error issue.
When the button is frozen, it will change back to the enabled state whenever the input is altered.
The address text input will get the focus when the "address" tab shows up.

Fixes #861