Move Qt Core examples under a common subdirectory

Qt Core examples were scattered into several subdirectories under
qtbase/examples. This caused an issue with the example manifest
file generated by QDoc; it expects to find all examples under a
common directory in order to produde correct paths to the example
.pro files. Qt Creator will not find the examples without a valid
manifest file.

This change moves the examples and edits the documentation files
accordingly.

Task-number: QTBUG-41963
Change-Id: I51d86782e0ba21c5c9bae5f15401ec774abe5cf8
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>