[qtquickcontrols-nemo] Initial import of a C++ based theme

This commit imports work done for having C++ based
theming, exporting a C++ Theme singleton object,
and C++ object for each component, starting with
the button.

It also includes the new theming system, using JSON,
and a tool to generate new components based on a
description file.
Comments and cleanups
Added support for groove and text field
Fixes