Switch ToolBarPageHeader to use the icon collapse behaviour from ActionToolBar

Summary:
This removes the icon collapse behaviour of ToolBarPageHeader in favour
of the implementation that is now implemented in ActionToolBar. It also
fixes a small performance regression that was introduced with the action
overflow behaviour.

Depends on D25177

Test Plan:
Kirigami Gallery still shows actions, the main/left/right actions change to
icons at the appropriate moments.

Reviewers: Kirigami, mart

Reviewed By: Kirigami, mart

Subscribers: plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D25178