Add option to draw border around yofi window

[Aleksanaa]

We often make the theme color of launchers the same as apps, and a border (with a different color) makes yofi more noticable. So a border with size and color options would be appreciated.

[l4l]

Sounds nice. The implementation is fairly easy:

• pass two config params;
• draw a border here with PathBuilder::arc, maybe with RoundedRect

If no one would take the issue I'm likely to implement it this/next weekend

[Aleksanaa]

Sounds nice. The implementation is fairly easy:

* pass two config params;

* draw a border [here](https://github.com/l4l/yofi/blob/master/src/draw/background.rs#L29) with `PathBuilder::arc`, maybe with [RoundedRect](https://github.com/l4l/yofi/blob/master/src/draw.rs#L87)

If no one would take the issue I'm likely to implement it this/next weekend

I'd like to implement a lot of things but I don't write rust, so what I can do is simply pointing things out🥲

[KuabeM]

I had a quick look and it seems pretty easy. I'd take a shot at this :)