Refactor brush input handling and ignore interactions over egui elements

[Megabjarne]

Before, if clicking somewhere on the brush selection toolbar, or on the egui inspector window, the brush system would still run and process the pointer event, leading to unintended dig designations.

Not sure if this approach is best practice, could do it with a run_if as well if that is more idiomatic

[tigerplush]

@Megabjarne Very nice! Do you think we could do without the #[allow(clippy::too_many_arguments)] ?

I was thinking that it's a very complex function already and maybe we could redesign it?

I definitely want to remove #[allow(clippy::single_match)] as soon as there are more actions to designate.
Although there is the argument to be had, that we can just fix it later xD

[Megabjarne]

I can see if i can restructure it to split and simplify it, would be benefitial for when more features are added later as well 👍

[Megabjarne]

Okay, separate system for registering inputs, that creates the higher level input events which are then used in the actual brush handler
I also considered adding rectangular selection here but figured this was enough for one PR