Remove RTC prefix from all names

Let's pull off the bandaid!

* Reduces studdering: webrtc.RTCTrack -> webrtc.Track
* Makes it easier to find types by editor autocomplete
* Makes code read more fluently (less repetition)

Since we're breaking the API in 2.0, our only chance to
do this is now.

Relates to #408

settingengine.go

@@ -25,7 +25,7 @@ type SettingEngine struct {
 
 // DetachDataChannels enables detaching data channels. When enabled
 // data channels have to be detached in the OnOpen callback using the
-// RTCDataChannel.Detach method.
+// DataChannel.Detach method.
 func (e *SettingEngine) DetachDataChannels() {
 	e.detach.DataChannels = true
 }