js: Use the aria-pressed attribute on fullscreen

This adds the `aria-pressed` attribute for the fullscreen button.  The
value toggles in the enterFullScreen/exitFullScreen methods.

Notably, the value is always `false` in Safari / iOS WebKit.  This is
because the WebKit's native fullscreen view disable access to the DOM
and all elements.  There is no way to interact or manipulate them from
the fullscreen view.  This should be fine, because that means any time
the user can interact with them, the pressed value should be false.

Closes: ADA-674

Author: awilfox

app/assets/javascripts/mediaelement-and-player.js

@@ -1317,7 +1317,7 @@ Object.assign(_player2.default.prototype, {
 		    fullscreenTitle = (0, _general.isString)(t.options.fullscreenText) ? t.options.fullscreenText : _i18n2.default.t('mejs.fullscreen'),
 		    fullscreenBtn = _document2.default.createElement('div');
 		fullscreenBtn.className = t.options.classPrefix + 'button ' + t.options.classPrefix + 'fullscreen-button';
-		fullscreenBtn.innerHTML = (0, _generate.generateControlButton)(t.id, fullscreenTitle, fullscreenTitle, '' + t.media.options.iconSprite, ['icon-fullscreen', 'icon-unfullscreen'], '' + t.options.classPrefix);
+		fullscreenBtn.innerHTML = (0, _generate.generateControlButton)(t.id, fullscreenTitle, fullscreenTitle, '' + t.media.options.iconSprite, ['icon-fullscreen', 'icon-unfullscreen'], '' + t.options.classPrefix, null, '', false);
 		t.addControlElement(fullscreenBtn, 'fullscreen');
 
 		fullscreenBtn.addEventListener('click', function () {
@@ -1476,6 +1476,7 @@ Object.assign(_player2.default.prototype, {
 		if (t.fullscreenBtn) {
 			(0, _dom.removeClass)(t.fullscreenBtn, t.options.classPrefix + 'fullscreen');
 			(0, _dom.addClass)(t.fullscreenBtn, t.options.classPrefix + 'unfullscreen');
+			t.fullscreenBtn.querySelector('button').setAttribute('aria-pressed', true);
 		}
 
 		t.setControlsSize();
@@ -1539,6 +1540,7 @@ Object.assign(_player2.default.prototype, {
 		if (t.fullscreenBtn) {
 			(0, _dom.removeClass)(t.fullscreenBtn, t.options.classPrefix + 'unfullscreen');
 			(0, _dom.addClass)(t.fullscreenBtn, t.options.classPrefix + 'fullscreen');
+			t.fullscreenBtn.querySelector('button').setAttribute('aria-pressed', false);
 		}
 
 		t.setControlsSize();