feat: add module class to the root elements of the modules

Previously, the only way to select all the module labels was with the
following kind of selector:
```css
.modules-left > widget > label,
.modules-center > widget > label,
.modules-right > widget > label {
    /* ... */
}
```
(and a matching block for the `box` containers).

Now, this can be expressed as
```css
label.module, box.module {
    /* ... */
}
```

include/AModule.hpp

@@ -11,6 +11,8 @@ namespace waybar {
 
 class AModule : public IModule {
  public:
+  static constexpr const char *MODULE_CLASS = "module";
+
   virtual ~AModule();
   auto update() -> void override;
   virtual auto refresh(int) -> void{};