Add ability to mark overlapping sidewalks with "covered=yes"

[TheAdventurer64]

Description

For cases like this, it would be cool to have the option to mark the overlapping sidewalk with covered=yes. tunnel=building_passage would be good as well.

Screenshots

How it would work:

Before:

After:

As you can tell by the photos, pressing the option to mark the sidewalk as covered would make the overlapping part split and marked with covered=yes.