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:
{F5763112}

{F5763111}

Reviewers: #gwenview, huoni, rkflx

Reviewed By: #gwenview, huoni, rkflx

Subscribers: richardbowen

Differential Revision: https://phabricator.kde.org/D11578