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 {
    /* ... */
}
```

src/modules/hyprland/workspaces.cpp

@@ -42,6 +42,7 @@ Workspaces::Workspaces(const std::string &id, const Bar &bar, const Json::Value
   if (!id.empty()) {
     m_box.get_style_context()->add_class(id);
   }
+  m_box.get_style_context()->add_class(MODULE_CLASS);
   event_box_.add(m_box);
 
   if (!gIPC) {