Adapt code to new crop terminology too

Summary:
D11578 changes UI strings from "Restrict to image ratio" to
"Preserve aspect ratio". Here we adapt the naming of functions and
config variables too, to lessen developer confusion.

Test Plan: Compile, grep for "restrict"

Reviewers: Gwenview, ngraham, huoni

Reviewed By: huoni

Differential Revision: https://phabricator.kde.org/D11589