Fix: Loop single videos via the player API (fixes #45)

[swashbuck]

Fixes #45

Fix

• Loop now works when _media._loop is enabled. On the ENDED player state, if looping is enabled the player seeks to the start and plays again.
• Removed the &loop= URL parameter from the iframe src. It read the wrong config path (_loop instead of _media._loop) and, more importantly, URL-based looping suppresses the ENDED event the handler relies on. The single-video playlist= requirement is also avoided, which previously caused a blank playlist= and a "Video unavailable" error.

Testing

1. Add a YouTube component, set _media._loop to true, point _media._source at any embeddable video (ex. //www.youtube.com/embed/jNQXAC9IVRw )
2. Watch to the end. The video restarts and plays again.
3. Confirm the iframe src no longer contains loop= or playlist=, and there is no "Video unavailable" error.
4. With _media._loop set to false, the video stops at the end as before (and completes if _setCompletionOn is ended).

Note: _setCompletionOn: "ended" combined with _media._loop: true will never complete, as a looping video never reaches a final end. This is expected.

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