FlxCallbackPoint: Fix duplicate calls and call xy when x or y is set directly

[Geokureli]

Alternative to #3514 (Most code was copied from this)

Changing a point's x should trigger the x and xy callback, once (if non-null), same with y, any method that changes both should call all 3 callbacks, once.

• The y callback arg was never being used, so setting xy called the xy callback twice.
• Every operation function now calls set() rather than setting x and y individually.
• Setting x and y individually now calls the xy callback as well.
• Added a unit test to verify the number of sets for each method with each number of callbacks

[Geokureli]

CI is failing because of HaxeFoundation/hscript#155