Add livesync plugin

It attempts to produce a (nearly) gapless live stream by synchronizing its output to the running time and forwarding the next input buffer if its start is (nearly) flush with the end of the last output buffer.

If the input buffer is missing or too far in the future, it duplicates the last output buffer with adjusted timestamps. If it is operating on a raw audio stream, it will fill duplicate buffers with silence.

If an input buffer arrives too late, it is thrown away. If the last input buffer was accepted too long ago (according to late-threshold), a late input buffer is accepted anyway, but immediately considered a duplicate. Due to the silence-filling, this has no effect on audio, but video gets a "slideshow" effect instead of freezing completely.

The "many-repeats" property will be notified when this element has recently duplicated a lot of buffers or recovered from such a state.