[Feature] Optionally load libadwaita and use libadwaita widgets

[bbb651]

Allow optional support libadwaita, there are two levels for this:

1. Calling libadwaita::init at the start of the application
2. Using appropriate libadwaita widgets

Ideally this would be exposed as a use_libadwaita option under the GTK section, but this doesn't seem realistic since weak linking isn't support in rust currently and probably not anything soon, and dynamically loading manually seems very hard and seems like it wouldn't work at all with the rust wrapper for it. Having it as a cargo feature and selection it at compile time seems a lot simpler, and I can make a PR for level 1 if this is wanted.

[rharish101]

Sure, feel free to work on a PR 👍🏽