Add qdoc3 support for specifying dependencies in the example/demos manifest files

Add the \depends <arg> command as a metacommand in qdoc3. The parameter is the name of an example that the example being documented depends on. There can be a list of these \depends commands. They only appear in \example and \demo comments.

The relative path to prepend to the arg is ../../examples/. The attribute name to use in the manifest files is to be determined.