Examples and documentation should be moved to separate installer component

• when examples lie within a Qt tree, they only work with that Qt
• in the multi-platform packages (including online installer) you want examples & documentation independent from which Qt platform you install and which you don't
• duplicating examples and documentation through the platform components would be a resources waste, and leads to the strange behavior that you can't compile the same example source for multiple platforms

So it should be a separate component that installs separate from the Qt trees, and all qmakes must point their QT_INSTALL_DOCS and QT_INSTALL_EXAMPLES and QT_INSTALL_DEMOS to that location.