Incorrect manifest ids created

[pgundlach]

I add an image with AddImage(filename, destpath), but then the image name starts with an digit, I get an error from epubcheck.

For example when I add 07-savepages.png, the resulting line in package.opf in the manifest section is:

<item id="07-savepages.png" href="images/07-savepages.png" media-type="image/png"></item>

The 07-savepages.png is not a valid XML id.