Convert AxHost to use Pointers

[lonitra]

• Add StorageType enumto track what interface _instance is
• Remove T.Interface fields in AxHost and query interface on _instance based onStorageType
• Use StorageType.PropertyBag where appropriate. We never explicitly defined this before, but were essentially following the same pattern as the other storage cases
• Fix PropertyBag storage cases. This was likely never working due to this case always throwing in AxHost.DepersistControl
• Update methods in AxHost.State to use pointers instead of T.Interface

[JeremyKuhne]

Added a few comments. We probably should convert STG_* into a StorageType enum and add something for PropertyBag.

[JeremyKuhne]

All of these is checks need to go away. You need to query for the desired interface pointer.

[JeremyKuhne]

A few more comments

[JeremyKuhne]

Looks good!