Keep existing selection when changing facets

[kannes]

It would be great if selected features were not automatically purged when the facet search values or facet combination was changed. Currently any change clears the "Selected Features" list.

Best solution would be a checkbox "Keep selected features" above or below the list that would keep the selection not matter what the user does (except for changing the layer or re-opening the dialog).

This would allow a user to build a selection in several steps incrementally.