Restyle the music player bar

The bar is rounded and a little taller, with a round orange play button
and a restart button that lights up orange under the mouse. The icons sit
inside their buttons with room around them, and are one colour, cropped to
their shape. Progress fills in orange, the volume slider gets a speaker
icon, and the buttons say their shortcut keys in their tooltips.

Buttons grow a little under the mouse and shrink while held down, and the
play button pops when it switches between play and pause. ButtonBounce
holds these animations so other buttons can use them too.

icons/volume_light_icon.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="24" height="24" viewBox="0 0 24 24">
+  <path fill="#f9f9f9" d="M3 9.5v5c0 .55.45 1 1 1h3.2l4.1 3.6c.65.57 1.7.11 1.7-.76V5.66c0-.87-1.05-1.33-1.7-.76L7.2 8.5H4c-.55 0-1 .45-1 1z"/>
+  <path fill="none" stroke="#f9f9f9" stroke-width="2" stroke-linecap="round" d="M16 8.5a5 5 0 0 1 0 7M18.8 5.8a9 9 0 0 1 0 12.4"/>
+</svg>