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/ASlider.cpp

@@ -13,6 +13,7 @@ ASlider::ASlider(const Json::Value& config, const std::string& name, const std::
   if (!id.empty()) {
     scale_.get_style_context()->add_class(id);
   }
+  scale_.get_style_context()->add_class(MODULE_CLASS);
   event_box_.add(scale_);
   scale_.signal_value_changed().connect(sigc::mem_fun(*this, &ASlider::onValueChanged));