Add support for Unbound Generic Types

[worldbeater]

What kind of change does this PR introduce?

This PR adds support for .Events() extension methods generation for types like AvaloniaDictionary<TKey, TValue>.

What is the current behavior?

Currently, we generate code like AvaloniaDictionary<global::TKey, global::TValue> which apparently isn't a valid C# code, stumbled upon this when attempted to use Pharmacist in AvaloniaUI projects: https://github.com/reactiveui/Pharmacist/blob/main/src/Pharmacist.Tests/IntegrationTests/Approved/Avalonia.0.8.4.netstandard2.0.approved.txt#L284

What is the new behavior?

Now, the behavior described above is fixed, at least for that AvaloniaDictionary type. Now we use AvaloniaDictionary<TKey, TValue> as a private readonly field, and pass the generic type parameters to the generated RxAvaloniaDictionaryEvents<TKey, TValue> class. Also, we pass the parameters to the generated .Events() method, and now its signature looks like .Events<TKey, TValue>(). Also, there was an issue with "Rx" + name + "Events" classes that inherit from unbound generic types, and that should also be fixed now.

What might this PR break?

Hopefully nothing. Let's wait for a report from the CI?

[worldbeater]

Whoops, messed up with formatting in a few places. Fixing this now.