mobile: ThumbnailsBase: Rename properties to avoid overriding final properties of Kirigami
Summary: Fixes displaying the Drawer
Test Plan: Drawer is visible
Reviewers: Okular, ngraham
Reviewed By: ngraham
Subscribers: ngraham, okular-devel
Tags: Okular
Differential Revision: https://phabricator.kde.org/D21549