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/sni/tray.cpp

@@ -15,6 +15,7 @@ Tray::Tray(const std::string& id, const Bar& bar, const Json::Value& config)
   if (!id.empty()) {
     box_.get_style_context()->add_class(id);
   }
+  box_.get_style_context()->add_class(MODULE_CLASS);
   if (config_["spacing"].isUInt()) {
     box_.set_spacing(config_["spacing"].asUInt());
   }