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