mobile: Thumbnails: improve placement of searchField and label
Summary: Move the error label to the results grid view, which fixes the layout of the TextField.
Test Plan: TextField and label are still visible if no search result is found.
Reviewers: Okular, ngraham
Reviewed By: ngraham
Subscribers: ngraham, okular-devel
Tags: Okular
Differential Revision: https://phabricator.kde.org/D21551