External Media Sync / Backing Tracks - Allow mixing with Synthesizer

[Danielku15]

alphaTab has currently the limitation that it can only use either the synth or the external media or backing track.

The missing part is: having both together.

1. The external media or backing track would be the main media source.
2. alphaTab would respect the playback speed just as today it internally plays the song "virtually" to synchronize the cursor and other bits.
3. alphaTab would in parallel generate the audio via the synthesizer and play it through web audio.

[Danielku15]

Notes:

In this mode the behavior is similar to:

1. the synthesizer always stays active.
2. Additionally the backing track or external media player can be active modifying the current time position regularly
3. If no backing-track or external media is active, the synth just keeps playing (but respecting the sync points).

If the external media time updates indicate a drift of the time axis (simple threshold?), we need to seek the synthesizer. As the update should happen quite often, there is no need for a smooth correction.