Rename DeleteThumbnailCacheOnExit -> LowResourceUsageMode
Summary: DeleteThumbnailCacheOnExit has been promoted to a low-resource-usage mode. Reflect that change in the code.
Reviewers: Gwenview, ngraham
Reviewed By: Gwenview, ngraham
Subscribers: ngraham, Gwenview
Tags: Gwenview
Differential Revision: https://phabricator.kde.org/D21406