Set the columns in "Overview" mode to match the amount of pages if document is small

Summary:
FEATURE: 355283

The principle is simple. This patch enables overriding the default columns for Overview mode, if the document is small. For example, if document has 1 or 2 pages and the default columns in Okular settings is 3, Overview mode will enable 1 or 2 columns for better UX

Reviewers: Okular, rkflx, ngraham

Reviewed By: rkflx, ngraham

Subscribers: ltoscano, aacid, rkflx, ngraham

Tags: Okular

Differential Revision: https://phabricator.kde.org/D8385