[date-picker] Move toolbar buttons to overlay content light DOM

[web-padawan]

Describe your motivation

The vaadin-button elements for "Today" and "Cancel" buttons are currently in the overlay content shadow dom.
This means that they won't be affected by regular styles applied to vaadin-button tag name in global CSS.

Describe the solution you'd like

Move the button elements into a slot in vaadin-date-picker-overlay-content after #3904 is done.