Add GUI options for changing block models used.

As some block models are different in between versions, there should be a checkbox or other GUI control that allows them to be changed.

The issue names no file, test, or entry point. First clarify which block-model versions must be selectable and where the control belongs in the GUI; done means users can choose the required model version and the selected choice is applied correctly.

Written by the indexing model from the issue text.