Not possible to set negative elevation when editing OSM objects

Currently in the OSM edit dialog we have some fields classified as integers, these are rendered as combo boxes. At the momement they are all limited to >= 0. The result is that if you edit a place with a negative value for the "ele" tag, such as a sea trench, or places on land, but below sea level. They are set to zero in the view and it's not possible to go lower. We should probably introduce a distinction for rendering editable fields as signed or unsigned integers, as a tag like "population" we still want to prevent setting a negative value.