Do not expand same class twice in (quick) type hierarchy

[fedejeanne]

Fixes #1830

How to test

Expand an interface from here:

class ReproducerRedundantTypeHierarchy {

	interface I0 {}
	interface I1 extends I0 {}
	interface I2 extends I1, I0 {}
	interface I3 extends I2, I1 {}
	interface I4 extends I3, I2 {}
	interface I5 extends I4, I3 {}
	interface I6 extends I5, I4 {}
}

The (Quick) Type Hierarchy should not expand the same interface twice

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Author checklist

• I have thoroughly tested my changes
• The change is following the coding conventions
• I have signed the Eclipse Contributor Agreement (ECA)

[fedejeanne]

@jukzi this time without any required changes in AbstractTreeViewer

[fedejeanne]

I'll leave it as "Draft" until the tests pass.

[jukzi]

Its cool that you made it without new API. Wouldn' it be better to overload TypeHierarchyViewer.updateContent() such that you don't need to spread the implementation detail across several files?