Make examples downloadable from the example gallery

The current process to discover and work with examples is somewhat clunky: Users can navigate the example gallery, but in order to retrieve the example's full code, they need to either clone the pyside-setup repository or download the PySide wheels and look inside the site-packages directory. In both cases, they then need to traverse the file system to find the example they need.

The experience could be improved by providing a download link for each example in its respective page in the gallery. To get all examples, users could just clone the (or a new) repository.