Move the inline installer progress cancel button to the left side

Summary: This prevents the progress view from jumping around, and keeps it right-aligned just like the button it replaces

Test Plan: {F5860414}

Reviewers: apol, #discover_software_store

Reviewed By: apol, #discover_software_store

Subscribers: andreaska, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D13006