Constrain kickoff config button label size

Summary:
A better way to achieve center aligning is to use the alignment property
of QtQuick.Text.

This looks the same but because we now have left and right anchors text
gets elided properly rather than overflowing out the box.

Test Plan: Opened, looks identical

Reviewers: #plasma

Subscribers: jsalatas, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2901