Disable any functions that are not available when indexer is running

[danrubel]

If a function such as Open Callers or Open Declaration cannot be run because indexer is busy, then it should be grayed out until indexing is complete rather than allowing the user to select that action and confusing the user when nothing happens.

Other alternatives include:

* A butter bar, popup, or dialog notifying that the function is not available and allowing the user to wait or cancel

* Providing the user with partial results but clearly indicating to the user that the results are indeed partial. If possible, new results could be added as they are available.

[clayberg]

This is related to Issue #3428.

[pq]

The consensus seems to be for option #­2 (partial results with clear communication that work is in progress). Let's open tickets targeting specific features (e.g., Open Callers or Open Declarations) as needed.

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Added AsDesigned label.