Add a splash screen example

## What problem does this solve or what need does it fill?

I would like to make a desktop app with a splash screen to show some basic information (a logo, current version, etc.) of my software.

## What solution would you like?

Something like this:

```rs
pub struct WindowPlugin {
pub primary_window: Option,
pub splash_window: Option,
pub exit_condition: ExitCondition,
pub close_when_requested: bool,
}
```

## What alternative(s) have you considered?

[Tauri Splashscreen](https://v2.tauri.app/learn/splashscreen/)