Doc: Fix a number of documentation issues

This change fixes a number of documentation issues:
    - Rename index.html landing page (collides with Qt docs)
    - Fix navigation
    - Create Enginio QML module page to get correct documentation
      for import statement, fix linking for QML types
    - Fix subtopics for generated QCH files
    - Remove unused pages and files
    - Update documentation namespaces and URLs
    - Other minor fixes

Change-Id: I0a87ab6d0be7df53199e54f4ecd9f84ef30d8622
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Doc: Adding C++ classes page to list of modules.

-this page will then be listed in the "All Classes by Module" page.

Change-Id: I7804d9aa780d28224edfcb310a6e2d362d329faa
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Doc: Adding \since commands to the C++ and QML API.

-both classes and QML types should have \since.
-module introduced in Qt 5.3.

Change-Id: I12e0b1df4946dfdf16cd97567b5745b4b308d781
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Doc: Avoid linking to collision pages

C++ class and QML type references for EnginioModel use a shared
doc-include file to document a number of property/method.

This commit splits the model-query.qdocinc content in two, so
that we can pass a correctly disambiguated link as an argument
from the documentation that uses the include. This way, we
avoid linking to collision pages.

Also moves the 'see also' links to the actual property/method
documentation, fixes the \return commands, and does some other
cleanup.

Change-Id: Ia6e178f0ee91625e62289134eada7fb1edc85f62
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Fix documentation link to Enginio::AuthenticationState

Change-Id: I61d0c5dfdc9eb649b8be0f372dedf18220e51213
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>