Use more user-friendly terminology in the crop tool
Summary: Use the term "Preserve aspect ratio" to make the crop tool a bit more user-friendly.
Test Plan:
Reviewers: Gwenview, huoni, rkflx
Reviewed By: Gwenview, huoni, rkflx
Subscribers: richardbowen
Differential Revision: https://phabricator.kde.org/D11578