Check all FlxObjects for mouse events

Closes #1252. 

All FlxObjects + groups will now be checked when traversing groups rather than just FlxSprites and FlxGroups.  Sprite groups are treated as sprites + groups. Tilemaps are mouse-able now (you could `add()` them, but they wouldn't be checked). If you can `add()` any FlxObject to the mouse event manager, mouse events should be available for any FlxObject.

It was easier to `Std.is` than to compare `flixelType` since `OBJECT | TILEMAP | SPRITEGROUP` all extend FlxObject and `NONE | GROUP` don't.