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/custom.cpp

@@ -176,16 +176,18 @@ auto waybar::modules::Custom::update() -> void {
           }
         }
       }
-      auto classes = label_.get_style_context()->list_classes();
+      auto style = label_.get_style_context();
+      auto classes = style->list_classes();
       for (auto const& c : classes) {
         if (c == id_) continue;
-        label_.get_style_context()->remove_class(c);
+        style->remove_class(c);
       }
       for (auto const& c : class_) {
-        label_.get_style_context()->add_class(c);
+        style->add_class(c);
       }
-      label_.get_style_context()->add_class("flat");
-      label_.get_style_context()->add_class("text-button");
+      style->add_class("flat");
+      style->add_class("text-button");
+      style->add_class(MODULE_CLASS);
       event_box_.show();
     }
   }

src/modules/dwl/tags.cpp

@@ -113,6 +113,7 @@ Tags::Tags(const std::string &id, const waybar::Bar &bar, const Json::Value &con
   if (!id.empty()) {
     box_.get_style_context()->add_class(id);
   }
+  box_.get_style_context()->add_class(MODULE_CLASS);
   event_box_.add(box_);
 
   // Default to 9 tags, cap at 32

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) {

src/modules/image.cpp

@@ -7,6 +7,7 @@ waybar::modules::Image::Image(const std::string& id, const Json::Value& config)
   if (!id.empty()) {
     box_.get_style_context()->add_class(id);
   }
+  box_.get_style_context()->add_class(MODULE_CLASS);
   event_box_.add(box_);
 
   dp.emit();

src/modules/keyboard_state.cpp

@@ -132,6 +132,7 @@ waybar::modules::KeyboardState::KeyboardState(const std::string& id, const Bar&
   if (!id.empty()) {
     box_.get_style_context()->add_class(id);
   }
+  box_.get_style_context()->add_class(MODULE_CLASS);
   event_box_.add(box_);
 
   if (config_["device-path"].isString()) {

src/modules/river/tags.cpp

@@ -111,6 +111,7 @@ Tags::Tags(const std::string &id, const waybar::Bar &bar, const Json::Value &con
   if (!id.empty()) {
     box_.get_style_context()->add_class(id);
   }
+  box_.get_style_context()->add_class(MODULE_CLASS);
   event_box_.add(box_);
 
   // Default to 9 tags, cap at 32

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());
   }

src/modules/sway/workspaces.cpp

@@ -37,6 +37,7 @@ Workspaces::Workspaces(const std::string &id, const Bar &bar, const Json::Value
   if (!id.empty()) {
     box_.get_style_context()->add_class(id);
   }
+  box_.get_style_context()->add_class(MODULE_CLASS);
   event_box_.add(box_);
   ipc_.subscribe(R"(["workspace"])");
   ipc_.signal_event.connect(sigc::mem_fun(*this, &Workspaces::onEvent));

src/modules/wlr/taskbar.cpp

@@ -737,6 +737,7 @@ Taskbar::Taskbar(const std::string &id, const waybar::Bar &bar, const Json::Valu
   if (!id.empty()) {
     box_.get_style_context()->add_class(id);
   }
+  box_.get_style_context()->add_class(MODULE_CLASS);
   box_.get_style_context()->add_class("empty");
   event_box_.add(box_);
 

src/modules/wlr/workspace_manager.cpp

@@ -52,6 +52,7 @@ WorkspaceManager::WorkspaceManager(const std::string &id, const waybar::Bar &bar
   if (!id.empty()) {
     box_.get_style_context()->add_class(id);
   }
+  box_.get_style_context()->add_class(MODULE_CLASS);
   event_box_.add(box_);
 
   add_registry_listener(this);