LibWeb: Use UA shadow DOM for media elements' controls

Instead of using a custom paintable to draw the controls for video and
audio elements, we build them out of plain old HTML elements within a
shadow root.

This required a few hacks in the previous commits in order to allow a
replaced element to host children within a shadow root, but it's
fairly self-contained.

A big benefit is that we can drive all the UI updates off of plain old
DOM events (except the play button overlay on videos, which uses the
video element representation), so we can test our media and input event
handling more thoroughly. :^)

The control bar visibility is now more similar to how other browsers
handle it. It will show upon hovering over the element, but if the
cursor is kept still for more than a second, it will hide again. While
dragging, the controls remain visible, and will then hide after the
mouse button is released.

The icons have been redesigned from scratch, and the mute icon now
visualizes the volume level along with indicating the mute state.

Libraries/LibWeb/CSS/Default.css

@@ -729,6 +729,10 @@ video {
     object-fit: contain;
 }
 
+audio {
+    width: 300px;
+}
+
 /* 15.4.3 Attributes for embedded content and images
  * https://html.spec.whatwg.org/multipage/rendering.html#attributes-for-embedded-content-and-images
  */