Update some files to QQC2 and fix minor bug

Summary: The bug was that the Drawer use the `drawerOpen` properties now instead of just `open`. Also fix deprecation Kirigami.Label -> QQC2.Label.

Test Plan: Tested on my computer with the touchpad. I could potentialy try with my android phone, if needed.

Subscribers: okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D18637