Bring back Web Audio playback support

[katspaugh]

v7 switched away from Web Audio for the reasons described here: #2762 (comment)

However, we've seen a few use cases where Web Audio is preferrable over HTML audio:

• Precise control over current time and scheduling – impossible with HTML audio
• Problems with Variable Bit Rate audio – see Waveform run more and more out of sync to audio #2890
• Simulataneous playback of several audio tracks on iOS Safari – slow with HTML audio, fast with Web Audio
• Volume control on iOS Safari – disabled for MediaElement
• Certain audio encodings in Safari don't work

I have a prototype implemented that emulates the MediaElement API using a Web Audio BufferSourceNode. This could be used a shim for cases where MediaElement doesn't work.