improve position of Clock

Summary:
take into account the size of the empty area before the
user icons, make the clock disappear when there isn't
enough room for it anymore, as user icons
are more important. Same change in queue for
the lock screen as well

Test Plan:
tested in sddm test ui, resized the window to different sizes
to veryfy the layout looks sane in different conditions and
that disappears first the clock, then the user list and the
input fields always stay uncovered
replaces D5063

Reviewers: #plasma, graesslin, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D5126