Invoked by the AddinManager after the instance of the
Addin is instantiated. When invoked, The Addin
should register and menu items, and actions required for use during this
classes lifecycle. Addin authors should take care to ensure
that any extraneous initialization is not preformed on this method, and
any startup code that can be delayed until a later time is delayed, as
the Addin's are synchronously initialized during the
startup of the IDE, and each Addin has the potential to
negatively impact the startup time of the product.