Bug 845173 - [MMS][Utility] smil document generate and attchment extract utility for MMS message

This is only a interface and brief structure for smil document handling with current message app. We will separate the mms message message content into smil slides by this way:
```html
<!--Each slide page could only contains one media area and one text area at most bubble content example -->
<img ....  / >  <!-- slide 1 : page  with image and text -->
<text> string1 </text>

<img ....  / > <!-- slide 2: page only contain one image -->

<video ....  / > <!-- slide 3: page with video and text  -->
<text> string3 </text>
```
And please convert these information to slide array like this:
```javascript
var slideArray = [
{name: img1_name, blob: img1_blob, text: string1},
{name: img2_name, blob: img2_blob, text: null},
{name: video1_name, blob: video1_blob, text: string3}
]
```
This structure will be used in createMmsMessage(for creating mms message)/ extractMmsAttachment(return attachment data). If you have any other question or feedback please contact me ASAP, thanks!