Gas Station preset: move building field to fields

[arch0345]

According to Taginfo, a significant amount of features tagged with amenity=fuel also have a building=* tag (~23%).

dismiss because I overlooked that the building_area field sets the tag to yes by default (for fuel stations it should be empty by default, however)

[tyrasd]

Sorry, I overlooked that the building_area field sets the building tag to yes by default for new features. For fuel stations it should remain empty by default, however. I think a new field is needed for this particular case.

[tyrasd]

FYI: In 65f46b5, I've split the building_area field into one which has the default yes value (called building_area_yes) and one which doesn't have that default value.