Provide navigation/finding usages for SObjects/fields from both the stubs and the .object files

[Phil W]

I frequently find that I have an object file open (e.g. Account.object) and want to find usages of that object type or one of its fields in the Apex classes or triggers. To do so I have to navigate to some code where I know it is used then open the IC stub for the object. I can then perform the required searching.

The proposal is to:

1. Allow easy navigation between the stub and the object, and vice versa
2. Allow find usages against the object file, with context based on where the cursor is (e.g. outside a field definition it will search for usages of the object, inside a field definition it will search for usages of the object's field.

[Scott Wells]

Duplicate of #849.