Customize grid level

Two variants how it may be implemented, I'd be happy with either:

1. Grid level slider on the side of the map (the same way the draggable map corners mimick the resizeable objects, the grid height slider may appear on the side mimicking object height slider).

2. Dropdown with buildings "height groups" in the Export dialog. May contain normal origin heights by default (water, plateau…), but if a map contains rooms at custom levels - they should appear in the dropdown too.

The important part being, the viewport origin should also level with the grid.

Why: to export maps of different levels. The example is shown on the attached pictures of the same DAM, that are exported without grid, but have different grid levels (and viewport level). Previously this could be achieved with DAM editing.