sddm-theme: Un-break userSelected signal handling

It was lost during refactoring some time ago, but the intention was
to give focus to the next item in focus chain assuming that's either a
username or a password field.

This patch is a less flexible but also less messy and more direct way to
achieve practically the same thing. Additionally, explicit Tab focus
reason gives a loginButton a nice visual focus which one would expect
to see instead of no visual changes. It was tempting to directly call
startLogin when password is not required, but that would be surprising
when the signal itself is triggered by the Escape key.

CCBUG: 336368