Example source code is not installed alongside the examples

When building examples only the example binaries are installed. In Qt 5 both the source code for the examples and the binaries where installed.

We have detected this on Debian and also in Fedora.

This is a huge regression from Qt 5 in the sense that many, many users rely on the examples ' source code shipped alongside the binaries. In fact it's even more important for them to have the source code than the binaries themselves (but both the better, of course).

While the listing is just for Qt 6 base I am pretty sure this must be the situation for all the submodules.

Extra points: examples do not need to have RPATH set, libraries are going to be found in their right place.