Implement less volatile custom window positions

[LordMZTE]

currently, when giving a widget a custom position, i'ts completely messed up once the window gets resized. this could be fixed by making the position a ratio

[zeroeightysix]

This is a difficult problem, as if you were to use ratios, windows might overlap if you resize the minecraft window.

This is better fixed with docking, which might be available when we update imgui.

[LordMZTE]

Another solution to this could be to use the existing corner positioning system but additionally with X and Y offsets. This would allow for custom positioning and solve the issue of positions breaking when the window is resized.