Create skin part change handler.

[andy840119]

It's time to think how to create change handler for those skin parts...

[andy840119]

And because it might be hard to implement #977 right now.
So is there any way to implement change skin part handler(e.g: layout change handler) without core change handler?

[andy840119]

Also, where to place list of skin parts?
Using "XXXManager" seems not a good idea 🤔
.
maybe place into sub-screen and use ID?
Guess "XXXManager" should be removed eventually.

[andy840119]

Not really sure should use ApplyCurrentXXXChange for change all property in skin parts?
Because most of cases we should not need to trigger another skin part change after change the property.