Feature: imperatively condense app-header

[bennypowers]

Description

I would like a feature which enables me to condense the app-header manually.
For example:

_pageLoggedInChanged(page, loggedIn) {
  if (page === 'home' && !loggedIn) this.$.appHeader.condense();
}

or

// <app-header condensed="[[_computeAppHeaderCondensed(page, loggedIn)]]"></app-header>
_computeAppHeaderCondensed(page, loggedIn) {
  return (page === 'home' && !loggedIn);
}

Why not just use appHeader.scroll(0, SCROLL_POS)?

In #404 the proposed solution
is to imperatively scroll the page. This is only a half-solution for my case, in
which we require the app-header to remain condensed in all scrolling positions.

Who will use the feature?

Developers that want to create page layouts that always display a condensed header.

When will they use the feature?

When conditions require a condensed header, even at the top of the page.

Example

A login page fills the viewport height, without scrolling. Design has requested that the app-header appear condensed on the login page.

What is the user’s goal?

The user wants to control when and under what circumstances the header condenses,
specifically, she wants to cause the header to be condensed, even when scrolled
to top.