fix for incorrect final position when dragging legend

[bmaranville]

Bug fix

When layout.legend.xanchor !== 'left' or layout.legend.yanchor !== 'top', the draggable legend enabled by setting config.editable=true or config.edits.legendPosition=true does not work properly.

During the drag, the position updates correctly, but on release it snaps to a new position. This is because the width and height of the legend are not taken into account in the calculation of dragElement.align(...), with the second argument dv=0 for both xf and yf.

This PR addresses that by including the width and height of the legend in the calculation.

Examples

To reproduce the bug, show either of these figures and drag the legend:

{
  "data":
    [
      {
        "name":"A very long name indeed (though it could be longer)",
        "x":[1,2,3],
        "y":[2,3,4],
        "type":"scatter"
      }    
    ],
  "layout": 
    {
      "showlegend": true,
      "legend": {
        "x": 0.5,
        "y": 0.5,
        "xanchor": "center",
        "yanchor": "middle"
      }
    },
  "config":
    {
      "edits": {"legendPosition": true}
    }
}

{
  "data":
    [
      {
        "name":"A very long name indeed (though it could be longer)",
        "x":[1,2,3],
        "y":[2,3,4],
        "type":"scatter"
      }    
    ],
  "layout": 
    {
      "showlegend": true,
      "legend": {
        "x": 0.5,
        "y": 0.5,
        "xanchor": "left",
        "yanchor": "bottom"
      }
    },
  "config":
    {
      "edits": {"legendPosition": true}
    }
}

[archmoj]

Thanks for the PR!
💃