Add margin for float button aka MapFabs

[wgcv]

Add custom margin because some time overlap in the search textbox in the issue #88

LocationResult result = await showLocationPicker(
      context,
      "AIzaSyAfss1BCW8g4DtoLZF7vh_Xzz7AJJ5qzuc",
      initialCenter: LatLng(0, 0),
      automaticallyAnimateToCurrentLocation: true,
      myLocationButtonEnabled: true,
      requiredGPS: true,
      layersButtonEnabled: true,
      resultCardAlignment: Alignment.bottomCenter,
      desiredAccuracy: LocationAccuracy.best,
// custom buttonMargin
      buttonMargin: EdgeInsets.only(top: 150, right: 8),
    );