Integrate Egui UI

[zicklag]

We need to integrate Egui instead of RAUI for UI.

It should be trivial to just re-use the existing bevy egui plugin.

• [x] Integrate Egui plugin
• [x] Implement with 9-patch widget for Egui
• [x] Implement image-themed buttons
• [x] Implement retro-font handling

[zicklag]

OK, Egui is turning out way easier to use than RAUI!

The immediate mode nature get's rid of a lot of the complications of RAUI and makes modifying the ECS world so easy right inside the UI.

Also, I've just finished creating a 9-patch style frame for Egui:

It's super simple for the user to use, which is great. Now we've just got to get checkboxes, and buttons, and fonts.

[zicklag]

Got buttons working!

Decided to skip checkboxes for now, because I don't think we're going to use them right away.

That leaves font rendering left.

[zicklag]

It's all working!