Fix crashes in IntensityDataProjectionsWidget (#556)

The problem lied in unique pointer to QItemSelectionModel which was owned by IntensityDataProjectionsWidget and reset to a new instance on every context change.

It seems that this selection model is still referred somewhere even after switching context and thus should not be destroyed via std::unique_ptr::reset()

Now the owning scheme of selection model is the same as for "main" MaskContainerModel: they both are owned by MaskContainerItem and have the same livetime.

Resolves #556 (closed)