Poor Performance of Type Hierarchy View

[HeikoKlare]

Current Behavior

The type hierarchy view has bad performance when the type hierarchy is large. The root cause consists of two things:

1. Opening the hierarchy view is performed synchronously, thus the UI freezes when opening it.
2. Opening the view calculates the complete type hierarchy (which is canonically required for the view). This information is not cached in the AST.

Expected Behavior

Opening the type hierarchy view should not block the UI. The contents calculation should be performed asynchronously. If this is not (easily) possible, in a first step the calculation may be moved away from the UI thread while showing a progress dialog or the like, which allows to cancel the operation, to avoid that the UI blocks for a potentially long time.

Additional Information

This is related to #2.