Use a RowLayout for laying out ToolBarPageHeader

Summary:
This uses RowLayout to perform the layout of ToolBarPageHeader. It uses
the titleDelegate's Layout properties to control the layout of the Title
delegate.

Test Plan:
Run Kirigami Gallery.
All pages behave correctly regarding titles:

• Default view shows title + as many actions as fit into the window
• Resizing the window shows the actions disappearing until only the context action is left, after which the title starts eliding.
• Resizing back to a large size first un-elides the title, then immediately the actions start to appear again.

Reviewers: Kirigami, leinir, mart

Reviewed By: Kirigami, leinir, mart

Subscribers: leinir, plasma-devel

Tags: Kirigami

Differential Revision: https://phabricator.kde.org/D25141