Add UI event system

[RiscadoA]

Checklist

TODO, pls subdivide this into smaller issues if you pick this up

Problem

Currently, we have no event support in the UI plugin whatsoever, which is terrible for a UI system!
It is extremely important to get this done, because as is, it is impossible to use the UI for anything other than showing things.
Buttons etc, have to be implemented manually, by checking if the mouse position is within bounds.

Design

We should think very carefully about how this would be implemented.
We need at least the following features:

• mouse click detection
• mouse hover (enter & leave) detection

Events should also bubble up if they are not handled. I.e., if the text of a box inside a button is clicked, and the box doesn't handle the event, it should be passed to the button itself. It is unclear how we would do this in ECS.

It would be cool to also be able to control UI with gamepad input.
If you're picking this up, make sure to investigate how other UI event systems are designed!