Improve delete part feature for polygon

The delete part is currently implemented using snapping. This means that for a multipart polygon, a part has to be deleted by clicking on the boundary. I think it is more intuitive for the user to simply click anywhere in the polygon to delete a part.

I also removed the cross that is drawn when pressing the mouse. It is only visible between pressing and releasing. I don't think it is very useful as the normal operation to delete a part does not involve any dragging, so this cross only produces a small flickering of the cursor.

The position of mouse used to decide which part to delete is the position of the mouse when pressed. Is it actually better than the position of the mouse when released? The part I added for polygons is using the position of the mouse released, but I can change in one way or another.

I also experimented with rubberband showing the part under the mouse before clicking, but it seems to give quite poor performance to calculate this at each canvasMoveEvent. Do you think it is possible to do it efficiently enough? It is not essential but would be quite nice.