sddm-theme: Focus the password field if enter pressed in the username field

Summary: Currently nothing happens when pressing enter in the username field.

Test Plan:
Entered username, pressed enter.
Now the cursor is in the password field.

Reviewers: Plasma, broulik, davidedmundson

Reviewed By: Plasma, broulik, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D12510