Implementing new `Parent` widgets sucks

[Osspial]

There's a lot of boilerplate when implementing Parent for a custom widget. There should be some way to improve on the current state of things, whether that be macros or some other solution.

[Osspial]

The best solution is probably to move WidgetContainer and its derive macro into derin_core and have the Parent trait expose that instead of all its current methods. That also aligns with Derin's goal of letting people create GUIs with minimal new syntax/non-derive macros.