[sddm-theme] Style the session and keyboard layout selectors to be more Breeze

Summary:
The session selector's and keyboard layout's menus are now styled to resemble the Breeze theme rather than the default QML style.

BUG: 408702

Test Plan: Ensure the styling looks good regardless of various conditions. (resolution, dpi, distro, amount of sessions, etc.)

Reviewers: #vdg, filipf

Reviewed By: #vdg, filipf

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21794