Article viewer: Zoom has to be applied after page starts to load

Summary:
Arch Linux package updates articles are currently being rendered without zoom.

I have tested moving the call to setZoomFactor to later QWebEnginePage signals to find the optimal place to put it.

loadFinished: Renders zoomed, but only after the article is fully loaded.
loadStarted: Renders without zoom.
loadProgress: Renders zoomed.

This patch implements the loadProgress option.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, dvratil, #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D12454