Added 'active' attribute to FlxButton

Based of findings here: https://github.com/HaxeFlixel/flixel/pull/1227
Would allow setting "active" for an FlxButton in the XML. Helpful if you want to prevent a login button from being pressed until the fields are filled. 
Also updated the documentation (added active and visible in the button attributes list).
However, as of right now, visible and active only work for buttons. 
This could be changed to be applicable to all widgets in load() right after it adds the id, it could look for visible and active and apply them:
```
//531
          var thing_id:String = U.xml_str(obj.x, "id", true);
          var isVis:Bool = U.xml_bool(data.x, "visible", true);
          var isActive:Bool = U.xml_bool(data.x, "active", true);
//555
            if (thing_id != null && thing_id != "") {
              _asset_index.set(thing_id, thing);
     
              //The widget id can be used in getEvent-handlers.
              thing.id = thing_id;
            }
           thing.visible = isVis;
           thing.active = isActive;

```
Your call.