Start the text generation request via a queued connection

Summary:
so that pixmap generation gets a chance to start before the text generation

This makes that on files where both the text generation and the pixmap generation
are slow at least we see something "as fast as possible" since only either text generation
or pixmap generation threads run at the same time

Reviewers: #okular, mlaurent

Reviewed By: mlaurent

Subscribers: michaelweghorn, dvratil, mlaurent, #okular

Tags: #okular

Differential Revision: https://phabricator.kde.org/D8378