wlr/taskbar: add justify option for Gtk::Box horizontal alignment

The `wlr/taskbar` module had no way to control its container's horizontal
alignment. This change adds a `justify` config key (left|center|right) that
maps to Gtk::Align::START/CENTER/END and is applied via set_halign() on the
module's Gtk::Box. Default behavior remains unchanged (left).

man/waybar-wlr-taskbar.5.scd

@@ -57,6 +57,10 @@ Addressed by *wlr/taskbar*
 	default: false ++
 	If set to true, group tasks by their app_id. Cannot be used with 'active-first'.
 
+*justify*: ++
+	typeof: string ++
+	The alignment of the text within the module's box, allowing options 'left', 'right', or 'center' to define the positioning.
+
 *on-click*: ++
 	typeof: string ++
 	The action which should be triggered when clicking on the application button with the left mouse button.

src/modules/wlr/taskbar.cpp

@@ -98,6 +98,17 @@ Task::Task(const waybar::Bar &bar, const Json::Value &config, Taskbar *tbar,
   content_.add(icon_);
   content_.add(text_after_);
 
+  if (config_["justify"].isString()) {
+    auto justify_str = config_["justify"].asString();
+    if (justify_str == "left") {
+      content_.set_halign(Gtk::ALIGN_START);
+    } else if (justify_str == "right") {
+      content_.set_halign(Gtk::ALIGN_END);
+    } else if (justify_str == "center") {
+      content_.set_halign(Gtk::ALIGN_CENTER);
+    }
+  }
+
   content_.show();
   button.add(content_);