Made the player root element focusable

This makes keyboard actions work reliably in the entire element.
Using -1 ensures it does not interfere with tab navigation.

multiplayer.js

@@ -1027,7 +1027,8 @@ input[type="range"]::-ms-track {
             this.shadowRoot.appendChild(icons.content.cloneNode(true));
 
             // initialize player skeleton
-            this._root = createElement('div', {'id': 'player-root'});
+            this._root = createElement('div', {'id': 'player-root',
+                                               'tabindex': -1});
 
             this._viewport = this._root.createChild('div', {'id': 'viewport'});
             const sidebar = this._root.createChild(