Add mxmlNewOpaquef and mxmlSetOpaquef functions.
Fix discussion writing code to look for opaque strings.
Fix dependencies for documentation generation.
- mxmlIndexNew() sort algorithm update to eliminate unnecessary
right-hand recursion.
- Combine printf() and puts() calls in mxmldoc to reduce the number
of calls and code size.
- mxmlSaveFile() now writes Unicode chars with fwrite for potential
speedup and modest size savings.
- mxmlEntityAddCallback() now returns an integer status so an app
knows when it was not possible to add an entity callback.
