Polygons with tags

[fernaper]

Right now we can add tags to polygons only if we attach a zone to them.
I want to be capable to add tags without a selection.

I think the easiest way to solve this problem is just to change the alpha value of the attached zone to 0, so it is completle invisible, while it exists (becouse, we need and want to have this rectangle, because it is necessary for selection the possition of each tag).

But I think, the better way to attack this problem is not saving the rectangle itself, just save the polygon, and make Utils.py generate the rectangle but only for the constraints algorithm.

Why the second idea could be better?
Because with the first option, we are drawing rectangles. Invisible ones, but at the end spending time in an unnecessary task.