Don't use application header

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:

After:

Reviewers: vkrause

Reviewed By: vkrause

Subscribers: kde-pim

Tags: KDE PIM, KDE Itinerary

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