Start of examples refactor

This is the general reorg of the examples directory structure, plus additional
guidelines.

calculator, animations and accessibility have been updated to the new standards
and tested, as an example.

Task-number: QTBUG-24133
Change-Id: I76c3b86751d3195ba2a5474ff23afb875765e9a4
Reviewed-by: Alan Alpert <alan.alpert@nokia.com>