Move to a proper router

The routing system in the app today is essentially boolean flags that toggle the visibility of components: https://github.com/divamgupta/diffusionbee-stable-diffusion-ui/blob/0155fc04b6859dbaba203fa3a0ebe151fd17c91c/electron_app/src/App.vue#L18-L22

That works okay-ish at that outer application frame level, but it becomes pretty tedious to switch routes from within components:

https://github.com/divamgupta/diffusionbee-stable-diffusion-ui/blob/a4a8659102960ae3825f9111c3d378fe16f58967/electron_app/src/components/ImageItem.vue#L66

Using a proper router would help us out here, and also allow for easier lazy loading of components. [Vue Router](https://router.vuejs.org/) is the most common choice, and it has a syntax e.g.:

```vue
Img2Img
```