[sddm-theme] Replace login button label with icon
Summary: This patch replaces the login button label "Log In" with an icon, places it to the right of the password field, and centers the combination of the password field and the login button. It also adjusts for the button's right border, which has a tendency to extend to the right and beyond the username input field by one pixel.
Test Plan:
Before:(user list screen):
After (uer list screen):
Before (user prompt screen):
After (user prompt screen):
Reviewers: VDG, Plasma, ngraham, filipf
Reviewed By: VDG, ngraham, filipf
Subscribers: davidedmundson, abetts, ngraham, filipf, plasma-devel
Tags: Plasma
Maniphest Tasks: T10325
Differential Revision: https://phabricator.kde.org/D19214