Add CMake target for installing sources of Qt examples

At the moment there is no CMake target for installing the sources of examples, one needs to copy examples manually to the installation directory. We do have things like install_docs, and these are convenient.

Having a CMake target doing that would be beneficial:

• examples need to be installed to show up in Qt Creator
• no need for every developer to figure out where the examples actually need to be installed, how
• no need for every developer to think up a terminal command for copying examples to the right directory (e.g. "git submodule foreach 'cp -r examples <installdir> || true'" - now, does that actually do TheRightThing?) with potential multiple trial and error cycles
• platform independent solution
• repeatable without potentially going through the whole thought process again