7.1.0 image cropper - removal of lots of duplicated code, fixes for various param options and some further unit tests

Now tested with all the following options
```C#
                 @{
                    //umbracoFile is image cropper property editor
                    var myImage = Umbraco.TypedMedia(1084);
                    var cropA = myImage.GetCropUrl("thumb");
                    var cropB = myImage.GetCropUrl("umbracoFile", "Thumb");
                    var cropB1 = myImage.GetCropUrl(200, 300);
                    var cropC = myImage.GetCropUrl(propertyAlias: "umbracoFile", cropAlias: "thumb", width: 100, height:100);
                    var cropC1 = myImage.GetCropUrl(propertyAlias: "umbracoFile", preferFocalPoint: true, useCropDimensions: true);
                    var cropD = myImage.GetCropUrl(propertyAlias: "umbracoFile", width: 100, height: 270);
                    var cropE = myImage.GetCropUrl(width: 100, height: 270, propertyAlias: "umbracoFile", preferFocalPoint: true);
                    
                    //uploadImage is cropper property editor
                    var mySpecial = Umbraco.TypedMedia(1086);
                    var mS1 = mySpecial.GetCropUrl(propertyAlias:"uploadImage", cropAlias:"Thumb", preferFocalPoint: true, useCropDimensions:true);
                    
                    //standard umbracoFile is Upload property editor
                    var old = Umbraco.TypedMedia(1081);
                    var oldCrop = old.GetCropUrl(width: 100, height: 270, imageCropMode: ImageCropMode.Crop, imageCropAnchor: ImageCropAnchor.Center);
                }
                <pre>@(myImage.GetPropertyValue<string>("umbracoFile"))</pre>
                <img src="@mS1" />                
                <img src="@cropA" />
                <img src="@cropB" />
                <img src="@cropB1" />
                <img src="@cropC" />
                <img src="@cropC1" />
                <img src="@cropD" />
                <img src="@cropE" />
                <img src="@oldCrop" />
```