4.x: Add the RetryWhen operator

[akarnokd]

This PR adds the Observable.RetryWhen extension method and operator. One can use the handler to have the Exception of the main source turned into a regular value and reacted to via an IObservable flow. This allows performing timed and/or retries by manipulating those errors via standard Rx operators.

[akarnokd]

The build failed for some reason (or I can't see the reason on the VSTS dashboard).

[clairernovotny]

I don't see anything either, which is weird. touching the PR should trigger another build.

[akarnokd]

We are getting somewhere:

[xUnit.net 00:00:31.1080629] ReactiveTests.Tests.QbservableTest.Qbservable_Observable_Parity [FAIL]
       Missing Qbservable operator: RetryWhen
       Expected: True
       Actual:   False
       Stack Trace:
            at ReactiveTests.Tests.QbservableTest.Qbservable_Observable_Parity()

I can see a Qbservable.Generated.cs file. How is it regenerated? Do I have to manually add a partial class like Qbservable.Joins.cs?

[clairernovotny]

I'll need to take a look and remember :)

[akarnokd]

There is this HomoIcon program that should generate those files but it hasn't been updated to support the new project structuring.