Add playback speed control with pitch correction

[carterprince]

This patch adds playback speed control (0.5x to 2.0x) with optional pitch correction.

Features:

• Requires gst-plugins-bad for the pitch element
• Gracefully degrades if plugin is not installed
• Adds UI controls in preferences
• Preserves speed setting when toggling pitch correction

Screenshot:

I'd be happy to make any requested changes.

[carterprince]

With gapless playback enabled, the seek bar would behave erratically after a track transition. I pushed a fix that resolves this by resetting the pipeline's timing when a new song starts.

While this technically introduces a discontinuity, the impact should be imperceptible. On my machine, the entire asynchronous seek operation completes in about 1-2ms, since the next track is already loaded and prerolled.

This seems like the best trade-off but I'm definitely open to any suggestions/feedback.