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/ALabel.cpp

@@ -20,6 +20,7 @@ ALabel::ALabel(const Json::Value& config, const std::string& name, const std::st
   if (!id.empty()) {
     label_.get_style_context()->add_class(id);
   }
+  label_.get_style_context()->add_class(MODULE_CLASS);
   event_box_.add(label_);
   if (config_["max-length"].isUInt()) {
     label_.set_max_width_chars(config_["max-length"].asInt());