Implement polygon filter

[zsoltk]

Documentation: Overpass_QL#By_polygon

Implementation guidelines:

• create class OverpassPolygonFilter extends AbstractOverpassSubQuery
• add method public OverpassPolygonFilter polygon() to OverpassFilterQuery (by the same logic as OverpassFilterQuery is embedded in OverpassQuery.filterQuery()
• add method public OverpassPolygonFilter addPoint(double lat, double lon) to OverpassPolygonFilter (should return the query object itself by the logic applied everywhere else in the Query classes)

When done, it should be possible to use the polygon filter like:

new OverpassQuery()
    .filterQuery()
        .node()
        .polygon()
            .addPoint(lat1, lon1)
            .addPoint(lat2, lon2)
            .addPoint(lat3, lon3)
            // ...
        .end()
    .end()
    .output(100)
    .build()
;