shell: Add input space validation when adding a server

Add input validation and visual style when error occurs.
Avoid resizing the dialog when error messages show up.
Remove get_focus and lose_focus functions.
Add some restrictions for empty host name in case "Next" button is enabled accidentally.

Fixes #943
Closes #957
Reviewed-by: Stef Walter <stefw@redhat.com>
shell: Remove extra spaces in cockpit-setup.js

Reviewed-by: Stef Walter <stefw@redhat.com>