Implement embedded/data image support

I'd like to insist that the team get around to implementing the Collada 1.5 HEX element for embedded images. I think 1.4.1 has DATA or something like this instead. From COLLADAFWImage.h:

```
/** Contains a sequence of hexadecimal encoded binary octets composing the embedded
image data.*/
// Disable until we really need it to avoid problems with copy/assignemnet operators
//CharacterArray mData;
```

How do we define "really need it" here? Can one person's request be enough? Here (https://developer.blender.org/T47999) is a link that includes a link to a DAE file with an embedded image, if you can use one for testing purposes.

I can see about working on this myself if necessary. I am in the processes of reviving/upending the original Collada-DOM project, and I need a peer application for my purposes, and Blender is the only appropriate candidate that I know of. It is using OpenCollada, so it cannot be fixed in Blender.

Blender opens the file in the link. It says the file is invalid around the element. I think OpenCollada is probably vouching for its supposed validity, but I do not know for sure. I am working on getting debug symbols working with the Blender right now. I noticed though that this is commented out in the header.

Thank you. Also hello Remi. I recognize your name. I've been chatting with Mark Barnes somewhat the last few weeks. I hope you two are still tight.

Start with COLLADAFWImage.h and the linked DAE file containing an embedded image, then trace how OpenCOLLADA parses Collada image elements. Compare the Collada 1.5 HEX element with the 1.4.1 DATA form and identify the relevant parsing and image-data paths. Done means the sample embedded-image file is accepted and its image data is exposed correctly, with regression coverage if the existing test structure supports it.

Written by the indexing model from the issue text.