Onsets demo improvements

[jmarcosfer]

This PR includes several improvements to the onsets demo:

• URL reflects freesound ID and analysis parameters, which enables sharing results
• Added metadata and attribution included with the downloaded slices
• Swaps Freesound embedded player with a custom sound preview player, lowering the amount of requests by an order of magnitude
• Improved sound selection with "Confirm" button
• Made help text slightly less technical
• Allow slices playback with number keys 0 - 9
• Fix some web-audio-related inconsistencies to allow running in Safari
• Minor visual/layout improvements

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