Add scaletempo to prevent pitch changes on playback rate change

[leggettc18]

Work has actually already been done here, just opening this up for transparency. Previously the pipeline was set up for a multithreaded approach, and that may need to be added back in later, but for now it introduced more complexity than was needed. To add it in, I:

• Removed the audiosinkqueue and audiotee from the pipeline
• Add in the audioconvert, audioresample, and scaletempo Elements
• Set the scaletempo sinkpad to the audiobin's source pad.