Add initial support/api compatibility for libadwaita (GTK4)

[likeadoc]

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.