Summary: Using ApplicationHeader is deprecated, use implicit one instead. This makes Itinerary on desktop more "desktop-y", e.g. by putting actions from the context drawer into the top bar.

Test Plan:
Before:
{F6434202}

After:
{F6434203}

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: #kde_pim, #kde_itinerary

Differential Revision: https://phabricator.kde.org/D17061