IntervalHeap: Fix compile warning with mono 3.6.0

The nested class SyncIntervalHeap has access to the private field heap,
so its own heap field must be marked as new.
ListViewTestModule: Refactor Model class to inherit from BaseListModel

Mono 3.6.0 was warning about the Cleared and Reloaded event not being
used, so instead of adding a dummy implementation of OnCleared and
OnReloaded, we inherit from BaseListModel. This allows to make the Model
class even simpler.