mobile: fully port to QQC2

Summary: Remove all QtQuick Controls 1 imports and replace by QtQuick Controls 2

Test Plan: The mobile app behaves just as before

Reviewers: #okular, ngraham

Reviewed By: ngraham

Subscribers: ngraham, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D21675