[sddm-theme] Style the session and keyboard layout selectors to be more Breeze
The session selector's and keyboard layout's menus are now styled to resemble the Breeze theme rather than the default QML style.
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
Differential Revision: https://phabricator.kde.org/D21794