Adjust FlxMEM object order

[MSGhero]

I think there should be a way to adjust the mouse event order in FlxMEM at any time besides reorder().

Current issue in HaxeUI is that the event order depends on the order that core adds events, which can't be relied on it seems, as opposed to the draw order. I can't use reorder since there's no guarantee that the component is actually on the screen, and reorder would delete the register if it's not. Maybe a function that takes a group or sprite group and adjusts the order of its children's registers (basically reorder on a specific container)? Not really sure.

GitHub is helpfully reminding me that this is related to #1237, #1252, #2102, and #2104, aka the "how does FlxSpriteGroup work with mouse events?" issues.

[MSGhero]

@ianharrigan it's one of those things