Polygon improvements

[RezaeiDoust]

1. When a file that contains a polygon is imported, a child item is made under the main folder (that has the file name) with "polygon 1" as its name. this is not necessary. child item should be eliminated and the the filename should become the polygon's name.

2. if more than one polygon is selected and either "polygon intersection" or "polygon filter" is selected, the operation is performed on the first polygon and the rest of polygons are ignored. this is bug and needs to be fixed.