Update icon property of 'IconStyleData'.

[Dawood-Shahid]

Change the icon prop property of IconStyleData to 'Widget' instead of 'Icon', so that it can accept svg and png as well.

/// Creates an IconStyleData.
  const IconStyleData({
    this.icon = Widget,
    // this.icon = const Icon(Icons.arrow_drop_down),
    this.iconDisabledColor,
    this.iconEnabledColor,
    this.iconSize = 24,
    this.openMenuIcon,
  });