mobile: app: Fix TableOfContents

Summary:
- Correct QQC2 import
- Apply search field changes from Thumbnails here as well

Test Plan:
- Open https://openboard.ch/download/Tutoriel_OpenBoard_1.5EN.pdf with okularkirigami
- open the context drawer
- switch to the "Table of Contents" tab
- the ToC should load successfully

Reviewers: #okular, bshah

Reviewed By: bshah

Subscribers: bshah, okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D23066