Reserve a double slot for accessor properties

Reserve two slots in the internal class for accessor
properties. This opens up reducing the default storage
required per data property to one Value. In practice
this implies cutting the required memory in half.

Change-Id: Ifed897852bbdfd810018f0d6b049fca6690ead2c
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>
Use an array of Value's for Object::memberData

This cuts the memory required to store properties
in an object in half for the common case. Accessor
properties require two slots inside memberData,
but data properties only one.

Change-Id: I0bab1b88ca9ed5930abf065c77c89985b9ed5320
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>