Revise the restructured "Integrating QML and C++" docs

Revise the restructured "Integrating QML and C++" docs

These docs were yet to be cleaned up following the recent doc
restructure. This changes most of the the content in these sections
and includes some new docs and examples.

Currently all the code snippets are included inline. In a later patch,
these should be moved into the snippets/ directories and be included
using the \snippet command instead. Alternatively they can be moved
into examples/ to replace the BirthdayParty examples which are no
longer referenced in these docs as of this patch.

Task-number: QTBUG-26381

Change-Id: I94e3654e61476fad11fe81042d1bbe94fc649d06
Reviewed-by: Chris Adams <christopher.adams@nokia.com>