[SDDM Theme] Select all and focus password field on error

When logging in failed, focus the password field and select all, so you can start typing a new password thereafter.
I did the same in the lock screen.

There's a workaround for a bug which draws focus away from the password field on login
but never put it back if logging in failed.

Differential Revision: https://phabricator.kde.org/D2767