Allow multiple selections in Model and Set

Mikhail Svechnikovrequested to merge
m.0 into main

Set uses now list of selected indexes instead of single index.

QListView::selectionModel() is responsible for the visual side.

When selectionModel() emits selectionChanged, we update Set.

When Set emits setChanged, we update selectionModel().

To prevent infinite loops the blocker flag blockSelectionModel is used.

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