Since the virtual keyboard C++ interface is wrapped inside a
namespace, it is possible to get rid of "Declarative" name
in the class names, that would otherwise conflict with the
QML namespace in the documentation.

- Rename DeclarativeSettings to VirtualKeyboardSettings
- Remove "Declarative" from class names

The rationale for this change is that the name Declarative
refers to now obsolete QtQuick1 module. Also, the class names
are now the same in C++ and QML name spaces.

Change-Id: Ide050d47110443d894d95d35dddf0df5891587be
Reviewed-by: Mitch Curtis <mitch.curtis@theqtcompany.com>