Move Styles and Private under controls

Register the C++ private API from the controls plugin. The
private plugin is not needed anymore.

The Controls "submodules" Styles and Private are moved to
the controls subfolder in the sources tree.

Change-Id: I98358227c945b6cd1a8dbff9d07e6cad4f044277
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>