Fix URL Navigator margins

Summary:
The URL navigator was slightly narrower than full width until you
went to fullscreen and back, at which point it fixed itself. This
was because the fullscreen toolbars were visible on app launch,
but hidden when exiting fullscreen.

This patch simply hides them at launch.

Left before:

Left after:

Right before:

Right after:

Test Plan:
Open Gwenview. The left and right of the URL Navigator in Browse
should be flush with the edge (see screenshots).

Reviewers: Gwenview, rkflx

Reviewed By: Gwenview, rkflx

Differential Revision: https://phabricator.kde.org/D10881