Add support for Salesforce Connect external objects

[Scott Wells]

This is more of a placeholder for me, but while a generated OST does contain external objects with proper __x suffixes, I'm pretty sure that there are some places where IC assumes that all custom object names end with __c. Need to audit those and determine which need to be updated to understand the concept of external objects and treat them as custom objects in the IDE.

[Scott Wells]

It appears that external objects should not extend SObject, or at least should not inherit lifecycle fields from SObject.

[Scott Wells]

As of 1.7.9.1, the Id fields is the only field common to all SObjects. I took care of other external object-related changes a bit back, so this should be complete now.