Enhancement: Various VideoPress shortcode setting fixes

As @jeherve suggested in https://github.com/Automattic/jetpack/pull/3646#issuecomment-209419213
- [ ] Review whether Flash is needed in the dialog and what happens to other choices if it's ticked
- [ ] Reword autoplay label
- [x] Make video in editor responsive. Done in https://github.com/Automattic/jetpack/pull/3646/commits/ef7b3d3dfb9b8babafdd237c1926a3579d840669
- [ ] Implement a time field for the video time offset
- [ ] Move `tinyMCE.ui.FormItem.prototype.renderHtml` into `onopen` [see comment](https://github.com/Automattic/jetpack/pull/3646#discussion-diff-59735692).
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I played with this, and the improvements look nice. I think we could still improve a few other things, though:
### Flash

That old player doesn't include all the other options available in the modal, so it makes things a bit confusing. I shouldn't be able to select both "Open Source Codecs" and Flash, or Autoplay and Flash. If usage of the old flash player is not recommended, why adding it to the modal? Shouldn't we only leave it as a shortcode parameter instead, for legacy?
### Autoplay

Wouldn't it be less confusing to just call it "autoplay", or "autoplay this video", like YouTube and Vimeo do? "Autoplay video on page load seems more complicated than it should be.



I think "Loop" could also be simplified, like on Vimeo. Words like "page load" and "playback" will only make things more confusing for users imo.
### Layout issues

Although I use a theme including a post editor stylesheet (Twenty Sixteen), the VideoPress embed is larger than the editor. Is that to be expected?

### Video offset

Would there be a way to make this more user-friendly?


Right now, I can only enter seconds ("how many seconds is 1:35?"), and I can enter just about any random number.

Start by reviewing PR 3646 and its discussion, then inspect the VideoPress shortcode dialog and editor settings, including the tinyMCE.ui.FormItem.prototype.renderHtml path mentioned in the issue. Done means resolving the remaining Flash-option behavior, simplifying the autoplay and loop labels, adding a user-friendly video offset field, and confirming the editor layout remains responsive.

Written by the indexing model from the issue text.