[applet] Put the toolbar in the header area

Summary:
This patch implements part of the mockup at T10470 by putting the toolbar in a heading
area visually connected to the titlebar.

Test Plan:

Reviewers: VDG, Plasma, jgrulich, niccolove, manueljlin

Reviewed By: VDG, jgrulich, niccolove, manueljlin

Subscribers: plasma-devel

Tags: Plasma

Maniphest Tasks: T10470

Differential Revision: https://phabricator.kde.org/D29117