Add an option for a static crop

[mshenawy22]

First Thank you for the great work.
Can you please add an option to make the crop size static , just like with facebook profile picture , the user can only drag and move his picture inside a defined area but they can't change the size of the crop of this defined area.

Thanks and keep it up.

[mshenawy22]

I figured out a way to do it , but still will be nice if this feature is implemented natively in the package

use a stack and put inside it a container with a thick border that basically overrides the grid , this way the image will only move and there's no way to move the grid or change its size because its overriden by a container
Here's the code

```

Stack(

  children: <Widget>[


  Container(
    constraints: BoxConstraints.expand(
      height:300 ,
      width: 300,
    ),
    child :
    Crop.file(_sample, key: cropKey,
      scale : 2.0,
      aspectRatio:2,
      maximumScale : 2.0,
      alwaysShowGrid : false,
    ),
 ),
    Container(
      margin: EdgeInsets.symmetric(vertical: 70.0),
      decoration: BoxDecoration(
          border: Border.all(color: Colors.blueAccent, width: 12),
        //
      ),
      constraints: BoxConstraints.expand(
        height:170 ,
        width: 300,
      ),


    ),

)