[sddm-theme/lock screen] Improve appearance in software rendering mode (outlines and icons)

Summary: This patch improves the appearance of the sddm theme and the lock screen with software rendering in place (QT_QUICK_BACKEND=software) by adding an outline to the action buttons, correcting the error that prevented the username outline from appearing and by adding a background to the action buttons that are otherwise impossible to see on top of a very bright background in software rendering mode.

Test Plan:
Before (white background):
{F6673579}
After (white background):
{F6673580}
Before (Next):
{F6673581}
After (Next):
{F6673582}

Reviewers: #plasma, #vdg, ngraham, filipf

Reviewed By: #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T10325

Differential Revision: https://phabricator.kde.org/D19609