QmlDesigner.PropertyEditor: Fix CLANG warning

The constructor creates a true boolean QVariant. We have
to use fromValue instead.
Since Q_DECLARE_METATYPE does not work on template parameters
and there is no reason to use a template class, I turned
DesignerPropertyMap into a normal class.

Change-Id: I89d3ed727680fe0e62c94b67e7481fb202b26ca7
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Marco Bubke <marco.bubke@digia.com>