fix(playback rate menu): cycling rates via click (videojs#4486)

In 6.0, we changed the way that Menu Buttons were being created by
wrapping an actual button with a menu sibling in a container. The
playback rate menu also had a rate value display. In 6.x, this rate
value display ended up coming on top of the button itself and capturing
it's clicks. So, we want to remove pointer-events from the rate-value
display and resize the actual button to be the size of the control.

src/css/components/_playback-rate.scss

@@ -1,12 +1,17 @@
 // TODO: I feel like this should be a generic menu. Research later.
+.vjs-playback-rate > .vjs-menu-button,
 .vjs-playback-rate .vjs-playback-rate-value {
-  font-size: 1.5em;
-  line-height: 2;
   position: absolute;
   top: 0;
   left: 0;
   width: 100%;
   height: 100%;
+}
+
+.vjs-playback-rate .vjs-playback-rate-value {
+  pointer-events: none;
+  font-size: 1.5em;
+  line-height: 2;
   text-align: center;
 }