Add initial support/api compatibility for libadwaita (GTK4)

With the release of Gnome 42 nearing and standard applications slowly but surely switching from libhandy to libadwaita (e.g. EasyEffects) it would be nice if these applications are correctly themed.

Reference:
Overview of libadwaita and theming it
libadwaita API Specs

Current Behaviour:
Operating System: Current Arch Linux (Gnome 41)
"GTK_THEME=Dracula easyeffects"

Most things actually look ok - seems only some finetuning is neccessary (e.g. spacing seems off)

[caiocutrim]

+1 I would love to see this theme implemented in gnome 42 👍🏼, for a while I'm using the default adwaita dark theme variant and dracula theme in my vscode.

[benneti]

For people happy with just the color scheme there is this https://github.com/odziom91/libadwaita-themes ,
which fits in much better than the standard adwaita dark style, see:

[ran-dall]

For those of you waiting for LibAdwaita compatibility...

We're starting to look into this with @EliverLara in #208, as we'll have to move a few things around.

In the meantime, you can #200 to use the theme with LibAdwaita. I'll try to keep that PR in line with the changes made with LibAdwaita so you guys can keep up with the changes.

[Cornul11]

For those who want a more consistent look, for gtk-4.0 and libadwaita in general, you may as well, in the meanwhile, use this tool that I've created: GTK4ThemeAdapter. Albeit incomplete, it looks relatively well even now with the missing changes that are to be done in #208

[ran-dall]

@Cornul11 Great addition! Thank you for sharing! Gradiance is also a very good, semi-supported by GNOME option, as well.