Bug 1019841 - [Gallery] Update to use <gaia-header>

I'd be interested to know the performance implications of these pattern-matching selectors. Versus doing something like class="gaia-icon" data-icon="camera".

Also documentation is needed here because anyone using a class that begins with 'icon-' will see their text not work.  Since the module name is 'gaia-icons' consider using that as the class name prefix insted of just 'icon'.