QDoc: Implement automatic lookup of snippets (C++ vs. Python)

It would make sense for QDoc to automatically include a correct version of a \snippet, based on whether we're generating C++,QML, or Python documentation (i.e. select the correct file based on the file extension).

Possibly, either fall back to a default snippet file, or omit the snippet entirely, if the correct extension does not exist.

For Python, investigate whether it should be Shiboken or QDoc that handles the snippet selection.