Improve options UI for smaller resolutions

[paulirwin]

The options UI takes up way too much space on smaller monitors. We need to improve this UI.

[maximilien-noal]

Replace the theme with the default Fluent theme ? In compact mode.

https://docs.avaloniaui.net/docs/basics/user-interface/styling/themes/fluent => Changing theme density

But the Fluent theme looks worse on Mac.

There are a few 3rd party themes, like the Material theme:

https://github.com/AvaloniaCommunity/Material.Avalonia

[paulirwin]

I'm actually thinking about moving the options to a new window or modal behind i.e. a gear icon. I don't think the options need to be shown the entire time on the main screen, especially if users don't change them all that often.

[paulirwin]

Completed with #108