list-base: Drop get_view_ui()

Adrien Plazasrequested to merge
aplazas/nautilus:drop-get-view-ui into main

This adds virtual ListBase.set_enable_rubberband() and homogeneize the drag icon hotspot to always use the top-left corner.

This allows removing ListBase.get_view_ui() as it's only used to tap into the internals of the views in a convoluted way.

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